Someone could (and likely is) building a better Repl.it with Nix and Theia. It's fucking flabbergasting to me that everyone in this thread acts like Repl.it is some magical product.
And that's not even accounting for the half-dozen other production-quality VS-Code-in-browser projects.
Sorry, it's just not a unique space. Not surprising at an that an egostistic founder isn't handling it well.
Godbolt is Compiler Explorer, mainly to look at assembly output, but provides editing, benchmarking, and profiling of many languages and compiler versions. I think Godbolt delegates running code to some other site, don't recall who.
Cppreference.com is mainly a C++ and C language and library standards reference library, but provides sample programs and means to run them on various compiler versions.
That uses Coliru, which also provides edit, build, and run.