If you run a node (that accepts inbound connections) you can set up a monitor to check port 8333 for free at https://uptimerobot.com/
Discussion
What's the catch? I can easily monitor my node without the help of a third-party. Just use headscale and enjoy uninterrupted access to your node privately.
You can also check it with https://bitnodes.io/
In my experience bitnodes is less reliable and slower to alert, but yeah.
Works for tor?
Why on earth I'd give you any email to link it to my node address?!?
Must be your business idea, cause it's 125% retard :-/