At one point Signal had working backup & restore on Android, whereby you could keep your private keys and seamlessly move between phones, but they did away with that since a few users encountered bugs.
I really wish they hadn't axed it, it was a lot easier to use it than do an adb backup of the app & data.
Looking at that GitHub issue, Moxie appears to have popped in for half a second out of annoyance, then the main repo maintainer michaelkirk went and locked & limited the issue to just contributors.
The problem with backup & restore in Android was that it didn't work reliably. I know first-hand. If the feature isn't there, people won't rely on it, as they may do when it's there but doesn't work.
The thing with locking issues is that Signal issues get so much noise that the content will drown in it. I dislike not being able to add on-topic comments, too, but I understand how they got where they are.
It was a feature in the app to export an encrypted backup to external storage (encrypted with a passphrase of your choice), independent of OS-provided functions. It basically serialised and unserialised its own database.
I really wish they hadn't axed it, it was a lot easier to use it than do an adb backup of the app & data.
Looking at that GitHub issue, Moxie appears to have popped in for half a second out of annoyance, then the main repo maintainer michaelkirk went and locked & limited the issue to just contributors.