I have a site that I haven't done tests on for awhile, and just noticed that registration has NOT been working since WP4.6+.
The site has a custom registration page using Blogs MU theme, Buddypress, and Pro-Sites. It has worked fine up through WP 4.5.4 (and still works fine when I revert to that WP version).
WP 4.6 and 4.6.1 both break the blog registration process, e.g., a message says the selected blog name has already been used -- no matter what name is entered. So long as I revert to WP 4.5.4 all is well...
The registration page is highly customized. Basically I'm looking for help in identifying what changed in WP 4.6+ with regard to registrations and signups -- relative to WP 4.5.4 -- I'm presuming something got depreciated requiring alternative language.