Uptime Kuma - self-hosted monitoring tool
Services like uptimerobot.com make it easy to monitor whether websites are still working. The light versions of such SaaS solutions are free to use with some restrictions. Should you want more, you need to purchase a paid service.
Or set up website monitoring yourself with Uptime Kuma on your own server (or desktop :-) and monitor websites via a graphical interface. Peter uses Uptime Kuma on a Raspberry Pi to monitor his own and client websites. He shows how to install, set up and use Uptime Kuma via docker.