When I was making my IoT washing machine sensor, I used Blynk.io to push notifications to my phone but of course that service became commercialised and is no longer free. I could not find any nice easy to use alternative except for Telegram which is what I use now. This service looks exactly like what I would need and I appreciate the promise to keep it open source.
https://pushover.net/ Has existed for several years. It works great for me.
I set up Tasker on my phone such that when a specific low-priority (no visible/audible alert) message from pushover, containing only a hash key, is received it picks it up and does an HTTPS request to my self-hosted server to retrieve the actual notification text.
I have a bash/websocat client connected 24/7 to wss://client.pushover.net/ as well that I should probably put up on git for other people.