F-Droid is pretty strict about apps being as open-source as possible. I did have a fun incident related to this with the Mastodon app — they were like "oh you're using FCM so you must be using Google's proprietary library for that, let's slap a NonFreeDeps tag on it". Except I didn't use the Google library, I actually reverse engineered it and reimplemented the bare minimum to get tokens and receive push notifications. Their logic broke, but they were impressed :D
They decided to use "NonFreeNet" instead.