There's a lot of churn in what's used in green-field development, but browsers rarely drop support for old features, outside of extraordinary cases like the Flash plugin. Old web sites will usually work just fine in a modern browser.
Regarding old features, Manifest V2 -> Manifest V3 was so painful to some that they dropped Chrome.
I'm talking about compatibility for web content, not for browser extensions which are a more volatile environment.
Disagree. many old features have been dropped. The blink tag, for example.
but rarely in a backwards incompatible way - browsers still render very old sites