Sure. I use Whisperfish on Sailfish OS for Signal conversations. I hear the developer is quite positive about the support he gets from Signal about getting it up to speed. There was that famous blog post, I think in 2016, where they said they won't support third-party clients, but they got so much flac over it, they might have reconsidered.

Also, there is in the EU a Digital Markets Act being worked on. That will mean some networks that play a big part in society will need to allow third-party clients. https://ec.europa.eu/info/strategy/priorities-2019-2024/euro...