It’s some of that sugar-coated bitter medicine again. New Web platform features make life easier for developers – but they also push users to upgrade. And the cool new stuff often comes packaged alongside some user-hostile anti-features: like how they’re trying to hobble adblockers (“Manifest V3”) in the name of better extension sandboxing (itself a legitimate concern) – or how newer Ableton requires newer Windows, but newer Windows requires a cloud account – or how … etc etc.
The particular features mentioned here are used for specifying appearance, but not any deeper functionality. Hopefully, they apply primarily to the default theme – in which case some of the alternate themes as discussed at Colors and themes could make the site usable on “unsupported” browsers for a while longer.