These guys, RP1, claim to have a huge breakthrough in big virtual worlds. http://rp1.com​

"What we’ve achieved is a system capable of handling more concurrent users unsharded than any existing system
can handle at full capacity. By eliminating limitations of concurrency, we’ve made applications possible that sound more like science fiction than something coming in the next year.

The RP1 Platform accomplishes a true Metaverse using code that is as much as 500 times more efficient than anything known to exist. This means 500X fewer machines sitting in server farms taking up space, generating heat, and consuming electricity."

"Demo: Two Million People on 33 Servers. End of Q2, 2021" (from an older version of their web site).

Didn't happen. The current demo page says “We'll show you toward the end of Q2”, but they don't say what year.

The CEO is Sean Mann. Dean Abramson is the system designer. He claims to have done a video poker system which supported a large number of users on minimal hardware. Anybody heard of them?