This is pretty tragic. Unfortunately, Discourse development seems to be about always moving forwards, and a couple of times a year (it seems to me) there’s some kind of change which disenfranchises another wave of older devices or browsers.
You case might be one of those cases where using the stable version would help, a bit. But any kind of update policy leaves you vulnerable to the problem you see. And a policy of not updating leaves you vulnerable to running the older software.
I don’t see an answer, other than moving away from Discourse to some other software with a much more generous policy of supporting older devices and older browsers. I don’t know what that might be.
There’s also the likelihood of these older devices and browsers themselves being insecure. But that might matter little if they are used only as communication devices.