This guide will be short.
Basically, itch io has a bug in which tags that are not very popular don't display all of their games. One of those unpopular tags is spanking tag. As a result, searching by that tag shows only three games.
https://itch.io/games/tag-spanking
Which can be quite baffling, because some of the more popular spanking games like "Trick or treat", "Wet dream diary", Brat of the wild etc have that tag in more information. But clicking on that tag displays only 3 games and its not any of those.
So, what to do? Change url and add more popular tag in front of spanking.
For example:
https://itch.io/games/tag-erotic/tag-spanking
Now the search shows 53 games, including those mentioned above!
Adult and spanking shows 62
https://itch.io/games/tag-adult/tag-spanking
Edit: This is if you are not logged in. If you are logged in, go to Settings > Profile > Content — Show content marked as adult in search & browse