This isn’t really a Discourse issue, but I’ve decided that it’s time that I have something more formal in place to know when a site is down than to happen to notice or get an email from someone.
I’ve seen pingdom and uptimerobot.
Well, and here’s this: https://www.pcmag.com/article2/0,2817,2492697,00.asp. But most of those cost 10-100x what would make sense for me. See also Best Website Monitoring Software | 2019 Reviews of the Most Popular Systems for a list that seems too long to be useful; well, you can filter by various factors.