There is bad blood between F-Droid and the Signal devs. I don’t expect the app to ever appear on F-Droid. Signal’s developers are on record as preferring the Google Play store as the official distribution method, and even downloading the APK directly from the Signal website is something they tolerate only grudgingly.
Plus, some are predicting that forthcoming changes to Android – Google possibly mainstreaming its “advanced protection” model so that phone owners cannot install the F-Droid APK except through enabling ADB and pushing it to the phone from a computer over the command line – will further marginalize F-Droid.
If you browse the F-Droid website they actually mention that it's possible to use the .apk distributed by the developers, if the apk is reproducible. It's cumbersome and requires a lot of goodwill from both the developers and the F-Droid maintainers, but it's not impossible.
The thing is, people using F-Droid are most likely already aware that they can install the .apk directly from https://signal.org/android/apk/ so there's not much to gain (the .apk prompts the user when an update is available too).
Exactly, that was the reason Moxie gave for wanting to avoid F-Droid back in the day. Besides, I hear the .apk one can obtain from signal.org these days comes with an integrated update mechanism, anyway? As much as I am a fan of F-Droid, I really don't understand the criticism here. What advantages does F-Droid provide here?
Plus, some are predicting that forthcoming changes to Android – Google possibly mainstreaming its “advanced protection” model so that phone owners cannot install the F-Droid APK except through enabling ADB and pushing it to the phone from a computer over the command line – will further marginalize F-Droid.