💡 The Idea
At the moment it seems that notifications sent to Matrix are actually unencrypted. This is pretty confusing as even when room is set to be end-to-end encrypted all notification are handled as unencrypted messages and messages have this nasty warning sign next to them in Element:

If I've understood correctly, this would require implementing end-to-end encryption flow to Apprises Matrix plugin. This includes acquiring keys from other devices (user clients in Matrix terminology) and then encrypting messages.

Implementation guide here: https://matrix.org/docs/guides/end-to-end-encryption-implementation-guide

I'm not sure if there is something that could be blocking implementing this. If notifications were end-to-end encrypted it would make Matrix a superior place to receive notifications when true privacy is needed!