Hitcircles must never be hidden under slidertracks. Whether a hitcircle is partially or completely under a slidertrack, it is confusing for the player to read. Insane difficulties are the only exception to this rule.
I can't comment on the previous discussion.
This is a bad rule, and should be moved to guidelines (remember guidelines are not meant to be broken under normal circumstances). The exception would be the difficulty of the map. Insane = anything goes. Hard = partially hidden (75%?) allowed. Normal/Easy = slight overlaps only.
I don't have a problem so long as the number is visible, but if this stays a rule, it needs to be concrete. We can be vague with guidelines.
Edit:
Proposal for a guideline:
Avoid hiding objects underneath sliders in non-insane difficulties. Whether a hitcircle is partially or completely under a slidertrack, it is confusing for the player to read. Make sure the difficulty matches the frequency and magnitude of overlaps.
This should stop bad overlaps, especially in easy/normal. Any minor problems (like stacking a circle on a slider end on a 45 degree angle to the upper left) can be assessed during modding and experience.