Tactically, you might have a good answer here: by what means can people with old devices run new enough software to keep using Discourse.
It would be quite involved if one had a distributed community each of whom needed individual assistance.
Strategically, it would take effort, discipline, and testing to avoid disenfranchising older browsers (even if only by offering graceful degradation.) There would need to be a firm intention to avoid imposing an upgrade treadmill.
Or, perhaps the forum admin could decide that usability beats security, and stop upgrading - early, before their users lose access. Perhaps by keeping a very careful eye on release notes, upgrade until just before a breaking change. As we know, it’s impossibly difficult to downgrade Discourse.
(My own iPad dates from 2012, I think. It still works perfectly well for many purposes and I have no intention of replacing it. Perhaps one could argue that a 10 year old device is old enough to have fallen off the supported track. But as Apple’s offerings are both expensive and robust, there will be people using old ones, for a long time.)