There are changes that are only required when targeting 29, but nothing that won't work on the lower target. Neither is 29 required for Google Play, current requirement is target 28 for new updates. However they will require 29 after 2. November, 2020.
Unfortunately after building Gadgetbridge with this PR, Gadgetbridge no longer finds anything.
(Amazfit Bip, Mi Band 4, Fossil Hybrid HR and more are here on my desk)
That's suspicious, no errors in logcat or anything like that?
Just checking a theory, did you grant GB "always" location access or "only while in use"?
It's suspicious primarily because I really really can't see how that's possible, users have confirmed scanning does work on Android 4.4 (old BLE, old pairing), 5.0 (new* BLE, old pairing) and 10 (new BLE, new pairing).