I would call it "free developer experience" (using ADB), not "free sideloading".
If you want to send your app to a friend to download and install it directly on their phone (without using a computer with ADB), you need to be Google-approved and register your app first.
OP I was replying to presented his scenario of self developing an app he uses on his own personal device, my response was specifically in regards to that use case, not any hypothetical third party person.
I think you could use adb over tcp from a chroot in the phone itself? But that doesn't really make it easier from their standpoint, and this is just a step towards full lockdown which is coming.