Ajax validation on registration shortcode fields

PMPro checks this on form submit, but if you want to do this on the frontend via JS, you could hook into wp_footer() to add your code to lookup the username/email via AJAX. There might be some code out there that already does this (I wrote some in my book! https://github.com/bwawwp/bwawwp/blob/917fa62fbd9b91b98bf4f30289d26a387d34ac5a/chapter-09/example-06.php)

You could tweak that to work with PMPro. If you post this to the members forum, I’ll follow up there with the specific code. It would be fun to code up.