I would not call that arbitrary play store rules. Rather the general app sandbox has had tightened security, which affects api compatibility, thus play rejects

I would not call that arbitrary play store rules. 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.