I haven't posted an attachment without some sort of alt text in longer than I can remember, and I try to tag posts that might upset someone, though I don't use content warnings for that. But no, Fediverse clients have never had prominent "specify an alt text and consider content warning this" messages in their UI. Most people never notice the options, and have never done so. It wasn't built on that, any more than it is today. The only thing happening is people telling each other to do that stuff. But what you say only appears once, then gone forever, so it's a continuous uphill battle for you.

What I want is to be able to reply with tags, if the original poster can't or won't tag their disturbing shit appropriately. That'd make this a happier place.