There seem to be a lot of these services ("scrape X in order to generate an RSS/webhook/etc. change feed for X"), but I feel that I can't rely on any of them. For anything that I would need this particular service for, my highest-concern requirement is that I'm able able to "set and forget" the service for literal years at a time. But almost always, services like this just drop dead eventually; or they tweak their sc…
Visualping.io works pretty well as a SaaS.
They have an okay free tier (65 checks per month).
I just wish their pricing model was less check-based. I would use it a lot more if I could pay $10–20/month for unlimited checks, but that's not something they offer. Their pricing model scales up really aggressively compared to the underlying resource usage.