I don’t know what we can do about it, though, since our site admins aren’t really present day-to-day. They pay for our Discourse subscription, but beyond that they’re generally absentee.
Last year another poster had similar issues: Apparently I'm going to be kicked off the boards (Discourse upcoming incompatibility){New cheap PC posts moved out}
And a few of us hopped over to the Discourse official forums (meta.discourse.org) to complain about it. That resulted in a delay of a few months but no ongoing major changes. It was Discourse’s official stance (I’m paraphrasing here) that they needed to move fast to keep up with technological advancements. When challenged about why a simple text-based forum needs to keep evolving when it already does its job, especially if it meant alienating existing posters, they had no real answer. Backward compatibility is just not Discourse’s priority.
If we still had active site admins, we’d have the option of staying on older versions of Discourse (pros and cons there, such as security issues) or considering a different forum software or using Discourse plugins to help mitigate some of the issues. But because we don’t, we’re pretty much stuck with whatever Discourse upstream decides to do…
You could try to hide your browser’s user agent and see if that makes anything better. But that’s only a band-aid; if Discourse actually starts using features that your browser doesn’t support, it’s just a matter of time before it stops working completely (if it hasn’t already).