Well, evidently spambots are not the reason. The reason seems to be that I made a change to the game's page on itch.io.
My two games were both delisted after I made a change to their pages. For one of these games the only change was that I made a word bold. But after two days, they got listed again.
I just made another update to one of these games. This time, I made a bugfix-release, so a slightly more involved change. Immediately that game got delisted, while the other one remained listed. Also, the developer log which I made to announce the bugfix was made invisible.
It is strange. Before last month I could make changes and release new versions and make devlogs without any problems. Now it seems my games have a target on them. If they get listed again after a day or two it doesn't matter much, but I don't understand why I suddenly have to deal with this. If there is something I could do about it, I would, but without communication from itch.io I am veiled in darkness.