Usually we create an issue for merges that we saw but didn’t merge (typically on things that need more work than what was in MQ2 since there were a lot of “half working”) things that got implemented while we were working on releasing MQ. However, I don’t see an issue for that one. I can look back at the old code to see if I can find it but if you have a commit link that would jump start it.
One of the issues with See Invis mobs is that there are multiple ways for a mob to see invis and there’s not an easy way for you to know which method is causing them to see you. Some mobs can see invis but not SoS, for example.
Going to the “see invis list” idea — just because a mob can spawn with the potential to see invis doesn’t mean they WILL see invis, so you would have a lot of false positives with this method. You could make a potential list of see invis mobs but that seems like a lot of work for finding out if you MIGHT be seen.
The best way to know if a mob sees you is to con it.