I'll try to shed some light on this.
Are silenced slider tails allowed? Yes and no: it all depends on the usage. If you make the volume drop down to 5%, or 10%, or something else that's quite low, it means you want to hide the fact that a slider tail has sound: it's either missing in the audio, or it doesn't suit the song. I've seen many maps with silenced slider tails, and nearly all of them had it paired with overstretched sliders.
- Are overstretched sliders any good? Again, it depends on the usage. If you want to emphasize the fact that sound the slider is mapped to is continuous, and you don't want to make it end on the downbeat at the same time, it may be appropriate. However, some of the maps I had to work with or play in my spare time have whole sections consisting of nothing but sliders that are 3/4 long and placed in a row. You can't emphasize -everything-, overuse makes it play weird, and instead of playing intuitively and going in tact with 1/1, 1/2 or 1/4th snaps, you have to hold the key longer than necessary, and again, and again. It gets worse if a mapper likes to switch slider velocity back and forth, but that's a different story.
- Are they worth disqualifying? Honestly, we don't and never will have accurate metrics for this, because each case is different. In general, it looks like this plot, where X is how often overstretched sliders are used, and Y is a chance of the disqualification; the hotter the color, the worse usage is. Different people have this plot compressed or stretched in a different way, that's how things are.
Is 5% special? Not really, it's just the minimal level of volume you can achieve without having to edit .osu manually. You can use 10, 15%, soft sampleset, custom hitsounds or anything that makes slider tail inaudible and still get the same result/treatment, except that now you can say "hey, the tails are not silent, see, it's 6/7/8/9/10%". Howevr, it doesn't change the fact that they're barely audible, as if they didn't exist at all.
Last, but not least: there should
always be a feedback from a hit object. You are not allowed to completely silence
slider tail and slider track at the same time, as well as slider track and slider ticks
and other possible combinations, but you may just lower their volume instead, to the point where it is fitting and not noisy. If you have maps where some sliders are silenced almost completely (hello, No Dap) and they are ranked, this means they weren't checked strictly.
Oh, the maps Kibbleru refers to were probably disqualified due to poor use of overextended sliders, and 5% was only a nice addition to that.
So yeah, 5% is allowed, but please use it appropriately. We'll also probably start reviewing RC soon, as Loctav said in the beginning of Feb.