Rather the general app sandbox has had tightened security, which affects api compatibility, thus play rejects it to make sure it’s future proofed by author. In the meantime, during phase-out, sideloading works so old versions of app suddenly don’t stop working, but eventually that call will stop working.
Whether apps should be allowed to download and execute code out of the apk bundle itself is of course another political topic.
At least skimming the termux issue here is that it was done for SELinux policy changes in trying to improve security of application running on Android. Is that correct? Then the app store is trying to enforce this newer policy via forced API upgrades.
As you said, you can at least still side load if you would like?
I'm not saying there aren't issues. I'm just saying that you can install and run from gplay.