Web browsers with an end of life no longer receive security updates which not only makes them vulnerable to the end user, but I imagine its not safe for the server's which receive visits by them either.

Is it practical to block or enforce and notify the end user that their browser is unsafe and unsupported? If so, how would one achieve that? I don't know of any official or crowd-sourced listing with that information to parse and keep up to date. I'm aware that the practice can be custom built with User Agent parsing and feature detection for HTML5 enabled browsers.