Not Open Source but I’ve been using https://pushover.net/ for this use case since forever. The website still looks the same as on the day I signed up for it probably more than 10 years ago. Never had an issue and using that in all my scripts or cron jobs.
I had all sorts of nfty notification to ios issues despite lots of trial and error and a bunch of AI tokens. I also moved to pushover.
What's the advantage over, say, having a Telegram bot push you messages instead of notifications?