Essentially, Steam (check rule 15) and itch.io are getting rid of/delisting content that payment processors don't want to process. (It's mostly adult content.)

This website has more information on what will happen if you own a game that gets delisted, and how to get it back if at all. I would personally recommend downloading it and backing it up somewhere, especially for itch.io which is not listed on the website.

A video with link sources for its information in the description validates this. If you just want to click a link here instead of going to the video, here's one for Steam and one for itch.io.

If you would like to protest this, https://yellat.money/ works for anyone and Americans can take action at https://aclu.com/mastercard.

Please remember rule 5 and tag NSFW content as such. Discussion of NSFW's existence is fine, getting into details should be tagged.