Hey David,
we are constantly working on improving the a11y for the panel and made quite a lot of changes in Kirby 4 to eventually get to full compatibility. But to be honest with you, we are not there yet. If you or your clients run into a11y issues, they are to be considered bugs and you should definitely report them on Github. I’m sorry that I can’t simply point you at a certificate right now. I wish I could.