In a survey, via Settings > Publication & access > "Use CAPTCHA for survey access" I enabled a captcha for a survey. I got a comment back from a blind user who was using a screen reader, and notified us that the captcha equation was not visible/usable for them:
https://devin.masto.host/@devinprater/108797844560678426

The problem is that they don't see the equation, as no alt text to the image is provided. I guess adding the alt text would kinda defeat the purpose of the captcha (making it very easy for machines to read.

A solution might be to offer integration hcaptcha, which does have a way for visually impaired users to get through: https://www.hcaptcha.com/accessibility
Or, their solution (email verification + encrypted cookie) might be implemented directly in LimeSurvey.

I selected severity level 'partial block' as it impedes users that rely on screen readers from accessing the survey.