Finally found it...

https://developer.android.com/studio/publish/preparing#:~:text=For%20Google%20Play%20Store%2C%20all%20apps%20created,are%20required%20to%20use%20Play%20App%20Signing.

SO, as of August 2021 all new apps distributed on the Google Play Store must use Google App Signing.

Oh well, that's that, then.