Re: ADA Compliance?
Blind user here. While I don't use webmail at all now, I've seen both interfaces. It won't end up being an ADA problem for them. The basic HTML interface was much cleaner and easier to use, but the full interface isn't inaccessible per se, just much more annoying. The same is now true of Google Docs and Sheets: over a decade ago when I was forced to use this, they were basically unusable and I had to make a variety of excuses to get someone else to copy text I produced in it to avoid punching my computer in frustration. Nowadays when I've become forced to use it again after a wonderful period of not needing to, there's still a chance that I'll be frustrated, but I can't argue that it's unusable. When compared to basically any word processor, Docs is worse from an accessibility perspective (with the possible exception of the web version of Office365 but at least you normally have the option of locally installing it), but everything works enough that Google can claim that I haven't used it enough to be familiar with the interface. In their defense, that argument is probably true, as I have spent relatively little time learning to throw away the way interfaces work everywhere else and learn the ways they work there, and if I did I could probably be much faster whenever I'm doing something more complex than the basic editing I sometimes do.
Not to mention that there are various places that have applications that are much worse from an accessibility perspective and nothing ever happens to them. If the customer complaint doesn't make them change, I try to drop them and find a replacement. This is much easier when I'm using it personally than when it's my work's choice. Anyone with a captcha on your website that doesn't have an audio or text version, this is a one-time thing for me. I will get someone to read the captcha for me and then I'll dump you. I won't ask first.