Hats off to Xe for spending so much time on backwards compatibility, especially the tidbit about targeting Chrome 66.
I have a Mac from 2014 running Yosemite that I occasionally use to test for backwards compatibility in my own frontend code (for fun!). But IMO the best way to ensure compatibility is to use period-correct toolchains or toolchains where the pace of change is slower, like ClojureScript.
In the non-webdev realm, it's gobsmacking that you can put "FROM debian/eol:buzz" in a Dockerfile, and it will do what it says...