There does exist something similar to this: https://lemmy-status.org. It will eventually have an automatic list, but it is not implemented yet. They are currently adding instances in manually. The owner is @jelloeater85@lemmy.world, one of our infra people at lemmy.world. The website is not connected to lemmy.world by any means btw.
Oh sorry, didn’t see that 😅
I was also thinking that an opt-in or something similar would be nice. As overloading small project raspberries with a large monitoring website wouldn’t be that nice…