Probably not what you’re asking but sometimes companies are required to block insecure browser versions due to governmental certification requirements, and also because they could be held responsible if they allowed users to access their services with known insecure browsers and something then happened.

What we used to do at my job was to display a big red warning banner to users with outdated or unsupported browsers, without strictly preventing them from using the site. This resulted in some users (who apparently couldn’t read) submitting support tickets when the site didn’t work with their three year old browser, then getting angry when we wouldn’t “fix” the bug. More importantly tho, this led to a failure in an external audit. I won’t go into the details, but basically the only sensible way to meet that specific requirement was to block all outdated browsers (by user-agent matching)