I sent a mail to support, and I got a response a few days later. The response was that my games were indexed again (which they are), and there was a link which I had not checked before (because it is called "Search & Browse" and not "Indexing"): https://itch.io/docs/creators/getting-indexed. After checking out that page, one line struck me:
"Established sellers with no history of issues will skip this review step and will be indexed immediately."
So, evidently I had no "history of issues" a month ago, but now I do (unless itch has wiped it again, but I am not going to check that as I do not want to run the risk of getting deindexed again). The only "issue" I can imagine is that I reported 6 spam posts in one weekend. But those were really spam posts, so unless "issues" means "this creator is a target of spam posts," I do not see a reason why I should be automatically deindexed.
Anyway, I hope this circus has ended now. If it hasn't, I will notice with my next update and I will send out another ticket.
Interestingly, the spam bot which left all these posts has not been a problem for me anymore, while I did see those posts appear on other pages. So perhaps itch has found a way to stop them?