So yeah, this is quite an issue which is making a confusion between all mappers who use custom skins. The rule itself is fine, but it needs a little exception to avoid difficulties. Problem are for example sliderpoint30.png and sliderpoint10.png - Basically these are not used anymore by current skin version, they are transparent. This means if you skin it, it won't do anything, but only people with very old skins will see the difference. My idea is that skin should work perfectly as is supposed to without exceptions - Requiring people to have very old skin to make them see sliderpoints is kind of useless, however, there are cases where people are getting forced to still include these elements and you cannot always get a quick answer on "is it rankable or not", because some people won't know.Ranking Criteria wrote:
When skinning gameplay elements, complete sets of elements must be skinned. For instance, if you skin hitcircles, you must include a hitcircle.png, hitcircleoverlay.png, approachcircle.png, and a slider border color (this can be done by adding the line "SliderBorder: 255,255,255" (using RGB values) under the [Colours] section of each .osu file). The same logic applies to hitbursts, cursors, numbers, and anything else of this nature. If you didn't intend to skin an element in the set, then using the templateskin's version is okay. Setting a SkinPreference is also acceptable (e.g. forcing the default skin).
For this reason, I recommend to change it to this. It will allow people to skin only things they really need to skin and avoid from confusing mapper and modders.
When skinning gameplay elements, complete sets of elements must be skinned. For instance, if you skin hitcircles, you must include a hitcircle.png, hitcircleoverlay.png, approachcircle.png, and a slider border color (this can be done by adding the line "SliderBorder: 255,255,255" (using RGB values) under the [Colours] section of each .osu file). The same logic applies to hitbursts, cursors, numbers, and anything else of this nature. If you didn't intend to skin an element in the set, then using the templateskin's version is okay. Setting a SkinPreference is also acceptable (e.g. forcing the default skin). Skin elements tagged as transparent or not skinnable for beatmap shouldn't be included.