As a Seedvault sealion: actually if d2d is set (possible in A14+/U+ builds), it will backup the app userdata despite apps not allowing it. But if the App doesn’t use any of the inbuilt functions for backup Android offers (explicitly or implicitly) there’s going to be no backup.
So it’s less about being allowed than basic ability. But then it’s the same backup Google Android users get. Read some more on why Signal has technically Android backup, but it’s not the backup you thought and you should still setup the Apps own mechanism at Seedvault and Backups - #6 by tcecyk (still a Seedvault fan)