Assuming that the developer of Syncthing-Fork doesn't mind providing ID to Google, they shouldn't have an issue getting a signing key (we will see how this works in practice). They aren't doing anything objectionable to Google.

The bigger issue for third party apps will be things like Newpipe, where applying for a key will put the developers in danger of a lawsuit because it affects Google's business.

(The APK signing requirement is a fiasco, I'm not defending Google. Just pointing out that this app will probably not be as seriously impacted as others).

FWIW, adb install will continue to work: https://www.notebookcheck.net/Android-s-app-sideloading-bloc...