Presumably? But I don't know which ones those are, and I think whenever I've asked, nobody else seems to know either. So it's definitely not a central part of the Lemmyverse experience, even if perhaps it is more so in the Mastodon one?

Nevertheless, I always add alternative text, even if just to indicate that an image is present. But most people don't, and furthermore even if they did, I wouldn't know the difference, bc I can't see that text most of the time (except for posts now from PieFed, but not comments - and yeah, most posts here seem to be lacking the alternative text).

The connection is that people are barely aware that Alt text exists - it is not shown, hence they ignore it. If Lemmy were to show it, then it could become a greater part of the experience. Otherwise, why write it when >99.9% of the recipients will never know that it's there - and if there's a typo even you won't know that, bc you can't readily see it yourself? This is a case where the tools provided by Lemmy don't make alternative text usage popular bc it's very difficult to use. If the tools were better, then more people would use alternative text.