I guess I don't have many hosted side projects. Those I do, I don't put them on separate domains. I doubt people use or care, so I haven't added monitoring for individual projects specifically. I guess I could and should do that now that I established my own online monitoring without third-party free tier limitations.
I use Red Sift (free) for certificate monitoring, since Let's Encrypt dropped expiry warning notifications. Given it's automated, it's never been an issue for my self-managed projects, though.
For is-online/is-reachable monitoring, after free services I used became more restricted, and none matched my simple needs, I created Meercat Monitor, which runs on my server and checks some of my and work related projects/services. Simple checks scheduled checks, email notification, and a simple history and status view. Something simple enough and open source so I can audit and manage sources.