why is 180bpm picked as the standard for what measures are determined as too difficult for the player at any given level, if anything, pick 150 - it divides much better into certain bpms which are common at certain difficulty levels:
1/4 = 150
1/6 = 200
1/8 = 300
1/12 = 400
as opposed to:
1/4 = 180
1/6 = 270
1/8 = 360
1/12 = 540
which are significantly more illogical.

Slider velocity changes that alter the scrolling speed of the map are disallowed. An exception to this rule would be creating slider velocity changes to unify the scroll speed in BPM-variable maps. Scroll normalization is mandatory and must be done in maps with variable BPM.

would https://osu.ppy.sh/s/704987 be regarded as an exception to this rule? This should not be a rule; it should be a guideline, and, if that, this presumes a congruency between physical difficulty (what star rating assesses, and determines whether the difficulty is an easy/normal/whatever) and visual/mental difficulty, with regards to svs. There is no major correlation between these two factors. A chart that is harder physically does not mean it is better suited for harsher svs than a chart physically lighter. In fact, it's the complete opposite. Looking at some of the most popular harsh sv charts in the game (https://osu.ppy.sh/s/572000 - Coinage
https://osu.ppy.sh/s/723624 - Back Of The Yards
https://osu.ppy.sh/s/108708 - dEKA
https://osu.ppy.sh/s/591171 - Groundhog
https://osu.ppy.sh/s/415946 - Chloroplast Skin
https://osu.ppy.sh/s/673208 - Cycles)
you can see that most, if not all of them, fall around a low difficulty range relative to the ability necessary to learn the SVs. As such, no explicit rule should be included which dictates congruency between physical ability and SVs, I don't disagree with it being a guideline, but even-so, it's presented in a sense which implies that as your chart is harder physically, it becomes better to use harsher svs, when, in reality, it is the complete converse. this is harmful to new charters.

these are arbitrary and an extremely stupid way to dictate what is expected of players around x skill level. The best way to do this would involve looking at the rough patternical levels at x skill level, and dictating what is regarded as "too difficult" for players at that skill level.
if anything, this should entirely be up to bn discretion for what constitutes as too difficult for a difficulty level, but hey.

Note snappings of 1/6 and above should not be used. This is to make sure beginner players start off with an easy experience in the game.

what is absurd about this, is that absolutely no time frame (or note frame) is dictated. A 1/6 in isolation is absolutely no harder than a 1/1 in isolation, or a 1/16th in isolation, or a 1/10000 in isolation; context is extremely important with regards to difficulty as, on their own, every note is equal in physical difficulty.

What is not accounted here, at all, is patterning. Even if you were to include a time-frame for difficulty (lets say, 10 notes) the difference between a 1/6 10 note 2h trill and a 1/6 10 note 1h trill is huge, and this rule has absolutely no dictation between the two. What you'd need to do for this rule to make any sense, is to gather data on all common patterns, and assess what BPM they are roughly playable by by a player at this skill level.

I would say, above anything, that common sense, and the dictation of the BNG should decide what is playable, roughly, for players at whatever skill level.

There is almost no way to list what is expected from players at any given skill level due to the huge variation in patterning that can be brought. 250bpm jumpstream for just 1 measure can range from, say, 7dan to luminal; WITH NO SPECIFICITY TO PATTERNING, A RULE BASED ON WHAT SPEED IS EXPECTED FROM PLAYERS AT ANY GIVEN SKILL LEVEL IS, AT BEST, A SHOT IN THE DARK.

as such this rule should be removed; no specification on what snaps are or are not okay for players at any given skill level should be given, as, if anything, it would be almost entirely incorrect as it doesn't account for technical or physical difficulty. it is an arbitrary number - is 1/4 dense chordjacking okay when 1/6 streams are not? I believe almost everyone able to competently play would agree that a 1/6 stream is significantly easier than 1/4 dense chordjacking at the same bpm, yet this guideline dictates that the former is harder than the latter.

So, what should be done instead? Note that it is near-impossible (if not) to truly dictate what is capable by players at a given difficulty level. Even moreso, when the element dicating what is in which skill level is SR, which is absurdly flawed, and should not even be used as a metric for determining anything.

I suggest that, instead of providing objectivity in what BPM is playable, or not playable, by players at a given level (even with regards to patterns), you should explain the difficulty of patterning in the RC; explain concepts such as the difficulty of jacks, as they increase in length, they do not increase linearly but instead exponentially, and as such it should be understood that a chart utilising 2 note long jacks (minijacks) should not suddenly change into 3, or 4 note long jacks, as these are significantly more difficult, despite not being much of a change in patterning. Conveying this to charters reading RC will provide (atleast to some extent) a more competent understanding of the structure of patterning in comparison to other patterns at the same BPM.

Furthermore, explaining that 1H trills are more difficult than 2H trills, because 1H trilling increases in difficulty much faster than 2H trills (300bpm 2H trills are doable by most players at 9/10dan level; 300bpm 1H trills are beyond the peak of any top-tier player for any sustained period of time.) Would be significantly more useful than what is provided now. Then, a rough bpm (WITH LENIENCY) can be provided for each pattern, and then atleast, to some extent, a difficulty level acceptable for each difficulty can be determined.
As example:
Hard (As hard constitutes for 1st dan->4th dan)
Streams should be between 180bpm-230bpm, taking the length of a measure as baseline. Leniency should then be taken dependant on the length of the stream, longer streams increase the difficulty in stamina, so they should be slower for the same difficulty to apply (a 250 stream extended for a longer period of time would stretch into low/mid-insane) and vice versa. The same leniency should be applied to the technicality of patterns, the more technically challenging a pattern is, the slower it needs to be to exert the same overall difficulty, and vice versa. This leniency should be entirely down to the judgement of the bng.

With this; a basic understanding of difficulty can be presented in the RC.
(if you'd like more numbers for this, i can work them out roughly over the week, but you need to understand that SR is complete garbage, and that hard-level chordjacks may rate a chart as insane, and extra-level longjacks may rate a chart as hard. mania sr is unusably trash.)

Long term slider velocity changes should be in between 0.??x and 1.??x.

I also vehemently disagree with the nature of this rule - the numbers selected for these (0.90/1.05 for normal; 0.75/1.10 for hard; 0.5/1.10 for insane and extra) are completely arbitrary and make almost no consistent sense. Considering the gimmicky nature of SV usage and how it applies over other skillsets, to limit long-standing sv usage which may be contextually acceptable with the concept of the chart is absolutely absurd in my opinion. Like I say above, this takes the physical difficulty of a chart and compares it to the difficulty of the SVs - there are plenty of difficult SV charts that are not physically demanding; the physical aspects of a chart as assessed by SR should have absolutely nothing to do with what SVs are acceptable in your chart, ironically enough, as the physical difficulty of your chart increases. If anything, your SVs should get less effective on the song so as not to overwhelm the player or obscure other concepts at play - but this is tangential.

Inverse patterns must not be used. They are a very advanced type of pattern and that they require a lot of coordination to properly execute it.

the lack of explanation with regards to what constitutes an "advanced pattern" is painful here. What it should refer to, is to the technicality of a pattern - inverse patterns are technically complex, and, as such, are a higher difficulty than most patterns at the same bpm. This goes for OHTrills, polyrhythms, etc. which are technical. Why this rule is only presented in hard is beyond me, as, logically, it should apply to normal and easy inherently anyway but w/e.
Inverse patterns are also not the only type of "advanced pattern"; all it's specifying is that this pattern is more difficult than other patterns at the same bpm. as such, 50bpm inverse in a 200bpm stream chart is perfectly okay, because it is *that* much slower, the added technicality does not create an unfair difficulty spike. This rule is simply incorrectly labelling technical patterns and not accounting for difficulty properly.