I found a way to have itch.io show adult content in its search:
- Log in to itch.io.
- Go to your itch.io account settings and then check the check box labeled "Show content marked as adult in search & browse".
- Go to the bottom of the page and click "Save".
A small amount of adult content will be restored to your search when you are logged in. Perhaps this is content that passed itch.io's audit.
EDIT: I was alerted to the fact that the count of adult games that are searchable with the adult content option enabled is very low.