The previously reported and resolved issues of broken UI behaviour on legacy browsers (Latest Stack Exchange changes broke UI on Firefox Android), specifically, Firefox 68.11.0 (the last one before the brutal browser UI revamp) have returned. Also related report: Was support for the SeaMonkey browser just removed?
At some point in the last ~12-14 hours, I noticed the "Add a comment", the hamburger menu, Inbox, Achievement and Stack symbol (communities selector) drop-down expansions, up/down vote, bookmark, all stopped working again.
I raise this as a new issue since the original reports were against the mobile UI and have been presented with the common UI for some time now. I don't see any listed changes as having occurred in the time-frame in question in this answer, so sincerely hope it was an erroneous change.
I appreciate there was a lot of back and forth on the previous issues regarding backward compatibility for "unsupported browsers" but it was resolved once before. I'd ask the responses not go down the, "Just upgrade your browser or switch browsers path". It it wasn't broke, why break it? Yes, I accept the security risks of using an unsupported browser, just as I do driving an out of warranty car.
I put the question out again, if changes applied in the last day could be reviews and the once previously restored compatibility fix reapplied.
Additional observation: If clicking on the Inbox or Reputation icon, you end up on a page:
https://stackexchange.com/users/<number>/<name>?tab=inbox, the equivalent of clicking on the "Inbox | all items", which displays as a classic full page (like "Request Desktop Site", not fitted to the mobile display size (i.e.: tiny text, etc.). On that page, the drop-downs all still work as expected. Weird.
Also reported as broken are Silk browser (Amazon platform) and Waterfox Classic and MyPal (used on legacy OS - Win 7 /XP). Not everyone can afford a new PC just to get to S/E and if your device provider or OS doesn't let you install any old (i.e.: new browser), are you simply out of luck?