Tag "search" works just fine with the nsfw protection. It is just as age protected as it advertised - with the exception of certain tags.
Try it with this one (setting on and off)
https://itch.io/games/tag-science-fiction/tag-story-rich
And then try that one (setting on and off)
https://itch.io/games/tag-adult/tag-science-fiction/tag-story-rich
It is obviously an arbitrary list of tags and to my understanding they would just have to clear that arbitrary list to deal with this issue. Or did they hardcode that list somehow? It "works" for adult, erotic, eroge and some others, even non featured tags. Curously, it does not work for sex.
Also, I told them about the issue in April, almost a year ago. Request 152691.
And as a reminder for anyone wanting to try all this, if you have adult content released yourself, you cannot deactivate the adult setting for your account. Just browse for it without being logged in.