I just thought about your answer, and i hate to say this, but that's not an excuse.
how many players use unflinching? If i tell you that there's a bug affecting 100% of its users (supposing there's at least one enemy champion that slows), will it get fixed?
You may say "yes". Too bad it's still not fixed since release.
The worst bugs are those when they happen each game you have an item on. For example spellthief on Illaoi or deadman's on Mordekaiser. If you put that condition the bug happens 100% of their games.
ps: I told you this, even though i know you may not even be the one that should fix those bugs, but as a general riot behaviour i don't think the % of players getting affected by the issue impacts having the bug fixed or not