I’ve been using https://pushover.net/ for a couple years now and am very happy with it. It allows to create tokens for each application that should send notifications, which comes in very handy.
I think this would be a great node to have within n8n.
Under https://pushover.net/api you can see the requirements that API calls have. The basics would be the token (per application within pushover), the user (a user key) and a message. It has lots of optional parameters but a first version with just those would already be nice to have.