What is your target? I work in embedded systems: I fully expect software I write today to run for the next 100 years. This is based on experience, we have customers still using 75 year old machines to do real work (that is not collectors) and I like to think engineering has learning something about making better machines since then. I think that computers will forever be able to drop back to 10baseT ethernet, ipv4 and html 1.0.

Of course I don't expect everybody will be able to support those configurations. There are currently unknown security holes someplace in out old equipment so nobody sane will connect our old stuff to the public internet. Thus most developers are safe depending on users having something fairly new. However a few will target behind the firewall customers and they will be stuck supporting whatever that old machines supports.

You'll still need to support browsers from 2009 ;)