I'm honestly not sure. I have friends who were deindexed then reindexed, but I also know completely SFW work that has been deindexed too for unknown reasons since staff isn't responding to tickets anymore. It confuses me because they have been responsive in the past to moderation issues, but now, it is complete silence.
I was told specifically here by the forum moderator here (No Time to Play) that my game was left deindexed by staff on purpose, but I tried to compare my page to pages that were reindexed and saw nothing that would've alerted staff. I was also told this issue was raised to admins but that was a while ago now.
I don't know any deindexed dev who has successfully contacted support about this issue. The closest I've seen is Leafo clear up random misconceptions in the forums- I've never seen him actually talk to a dev that is actually affected.
Also regarding your theory- I've seen older games with regular high amounts of activity get deindexed after updating and my own game was deindexed while receiving a decent amount of activity too (And I updated it once). It really seems like staff is just firing shots in the dark.