I kind of wish these services were open source/core / had a self-hosted version, mainly because if they ever shut down, I wouldn't really know how to migrate out of them. But also, if they were open source/core, we'd get to add plugins / do all kinds of cool stuff on top as well.

Are there any services that are like this, but on top of say, Cloudflare Workers?

I use simple login and think it's great.

Another self-hostable option is AnonAddy https://github.com/anonaddy/anonaddy