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.
How does one block unsupported web browsers?
security;browsers;detection
A user with an insecure browser cannot harm a server. It's the other way around.And now....No! No! No! No! No! Never block a browser!If you build a site using Progressive Enhancement you can support every browser regardless of functional capabilities. A well-built website will be usable and accessible without CSS, JavaScript, and images. It may not look pretty and may not be as pleasant of an experience, but the content is still reachable. Then users who do support newer browser functionality will get an enhanced experience. It's quicker and easier to do then most web developers think.If for some reason you need to identify a browser never use user agents to identify them. They are easily spoofed and unreliable. Use feature detection instead.A nice side effect of this is your website will be be search engine optimized as well since they are essentially as capable as Netscape and MSIE 2.0.