Do not require use of Google Services for notifications
Do not force users to have to use Google Services for notifications. There are many reasons to choose not to have these services loaded and it is quite surprising to see Microsoft require Google for it to function.
Heinrich Ulbricht commented
Please yes provide an alternative. I'm currently on LineageOS without Google Services and notifications do not work.
Other messengers successfully implemented alternatives. Threema apparently provides a polling-based approach, Signal Messenger provides a WebSocket-based approach. There is the risk of battery drain with these approaches but at least with Signal Messenger you can profit from their experience which is all accessible on GitHub.
Linux support is great. Googleless AOSP support is the next even greater step.