This has long being the case for Opera browser in Presto age (before they switched to Chromium), but when Firefox' market share started to shrink considerably Google started giving them the same Opera treatment.

It's especially annoying for desktop versions of Google web offerings. When asked about it Google cite "low market share" as a reason for lack of support, but if one excludes mobile browsers FF still has a very respectable userbase on desktops.

And yes, just as in Opera days a User Agent switch magically lets you in and everything continues to work.

I honestly don't get why they continue with these practices. Chrome quickly approaches a monopoly position in browser space, and there are precedents what happens after. Besides, Mozilla and Google often align their interests in various standards groups and together push a common agenda for Web technology progress. I don't see why Google treats Mozilla as an enemy.

With old Presto Opera there was a valid stance of "it might not work and we are not going to test it". But with current Opera, or Vivaldi, and I'm sure many others, it should just work, since they are all Chromium based.

By the way, fun fact, old Opera had huge "compatibility" javascript where they fixed either general or specific issues that were result of developers not caring about the browser. Opera had over 50% share in certain markets at the time.