HagbardCeline I'm paranoid removing it from every app including the system ones. Is there any app that really needs it, or that we should avoid removing the permissions from?
You should not modify the permissions of any system apps. You can break things in unexpected ways by doing that. System apps are trusted. Don't tamper with them.
HagbardCeline As for the keyboard, I've been using the GBoard with disabled internet access. This should prevent from leaking, no?
No, this does not prevent data leakage. Gboard uses IPC with Play Services to send telemetry, not direct network access. The only way to stop Gboard from sending telemetry is by using the in-app privacy toggles.
The network permission is not an anti-data-exfiltration toggle. There are very many side channels that an app can use; it is not limited to IPC. If you don't think an app is trustworthy, you should not hand it sensitive data/permissions.