Things can remain backwards compatible forever. That is what any good standard does. Web standards and much else in software is sadly a complete mess where too few care about all the downsides of instability.
I am a bit worried because for many years I used plugins like SinglePage to save web pages as HTML. That is not exactly future-safe since every relase of Chromium or Firefox has a list of things that were deprecated (and a list of things that changed, that might or might not break rendering of old pages). Old saved pages will eventually begin to degrade and some might eventually be unreadable without having to mess with virtual machines to run old browsers.
I am a bit worried because for many years I used plugins like SinglePage to save web pages as HTML. That is not exactly future-safe since every relase of Chromium or Firefox has a list of things that were deprecated (and a list of things that changed, that might or might not break rendering of old pages). Old saved pages will eventually begin to degrade and some might eventually be unreadable without having to mess with virtual machines to run old browsers.