There is over a dozen "adult" tags. They do nothing as far as I am aware of, except break things and confuse people (there are over 10 tags all just saying it is adult content and you can only use 10 tags per game). See links below. There is a hidden marking that is not a tag, and used for negative filtering as described in the content setting.
Not all games are adult, even if they might look the part. Sex sells, after all. I am not even sure, if boobs alone would be enough to qualify for 18+ rating. You can check if a game does not has that marking by searching for the game name directly. Best while logged out (incognito modus provides that). If you can find it, it does not has the mark. (edit: actually, just copy the direct link and open in incognito window. If itch asks if you are 18+, it has the marking)
The gist of an unresolved issue in that thread is, that you can go to browse and select some of the adult tags, and the setting to not show the marked as adults is ignored.
https://itch.io/t/2890487/mature-and-adult-custom-warning-landing-page-check
My suggestion to actually make that hidden marking a searchable tag was largly ignored. :'-(
https://itch.io/t/2892152/make-marked-as-adult-a-searchable-tag