I know that the mastodon team are suggesting you turn on #hCaptcha for signups to your instance, to combat the spam wave provided by a rudimentary script that doesn't have the smarts to get around it.

I vaguely remember, however, that people have previously raised #accessibility concerns about it. Does anyone have any experience with the accessibility or lack there of, of signing up to instances that use this feature?

If it is indeed inaccessible, I don't consider this to be a reasonable solution.