A simple one, i believe that softwares must enforce them rather letting users doing it.
Here is some ideas :
- refuse "image post" without alt text.
- when you upload picture, you must provide their alt text, if there is no alt text, the picture will be deleted.
- hide user post based on alt text and remove post score if the alt text is bad.
- deactivate crosspost on posts who don't have alt text
It maybe harsh but i believe that's the only way to reach accessibility.
Edit :
- alt text autogen + a reminder bot will help users
- The app fedilab (mastodon) has some good rules on post with alt text.
- collaborative alt text.