Tor is as close as we might get. Onion services (especially v3) are arguably even more anonymous than browsing clear-web sites because there isn't a clear exit node. Tools like Ricochet and OnionShare allow for ethereal onion services that can open the door to some interesting usecases.

> Tor is not truly anonymous.

No low latency system will be anonymous in the face of a global passive adversary. Tor does not claim to protect against a global passive adversary.

> No low latency system will be anonymous in the face of a global passive adversary.

That isn't true. The trivial system is the one where everyone has a broadcast transmitter and transmits constantly even when there isn't a real message.

The implementation of that over the internet is full mesh, which is impractical, but it's a spectrum trading off anonymity for bandwidth and there are potentially interesting points along that spectrum between Tor and full mesh, where they use more bandwidth but then a global passive adversary can't learn anything. Something like virtual circuit switching could potentially work for example.

I guess you'd have to define truly anonymous

nothing is IMO. it's a function of raising the cost so it becomes too prohibitive for an adversary to track you. the only way to be "truly" anonymous online is not to go online (or to be truly anonymous irl is to stop being alive).