I know Gangplank is disabled, and I know they did some stuff for Anivia's Q. But the rest I have frankly never really seen. Part of bug-fixing, as well as troubleshooting of any kind, is to make sure you can reproduce the problem consistently so you can discover where said problem is coming from.
Plus to be fair to Riot's technical department, coding ANYTHING isn't exactly a walk in the park. Coding or fixing code in this game and making sure it doesn't make OTHER bugs happen? That's likely quite a challenge. As I understand it from a friend who does that kind of work consistently, very rarely is it a case of just 'oh this doesn't belong, I'll just delete that and everything will be fine.'
If you want them to fix the things, my suggestion would be to document each and every time they happen to you, and send a ticket in to Riot. You have to give them resources to work with in terms of where and when and under what circumstances these things happen. For example the Sion bug: you say it's just being killed by the Nexus laser, but what if the reason is really that they're killed by a Nexus laser while being marked by Grievous Wounds caused by Tristana? They could spend days, weeks, trying to replicate the bug and not get it, not realizing that the conditions aren't right to make the bug appear.
Bugs are annoying, but if you feel Riot should fix them just complaining about it only helps so much. They cannot make bricks without clay.
