Simplified, Thinnet is just RG-58, and RG-58 is just coax that has a characteristic impedance of 50 Ohms, and 50 Ohm coax works just about like any other impedance of coax as long as the termination impedance is correct (or the length between terminations is 1/4 wavelength), and the attenuation is not too severe.

One can send the whole RF spectrum down a single length of coax.

The little pinkish-copper wires in your wifi gear are just coax. SATA cables: Also coax. Uncompressed HD video over SDI? Also coax.

(But you asked about successes, not theory. I've run 5G cellular services through thinnet wire. Does that count?)

You can run 1GBps on it. There are companies that sell hardware for it, even specialized multi-port switches.

They are mostly used to retrofit the old CCTV infrastructure for newer PoE-based systems. It's typically called Ethernet-over-Coax. E.g.: https://www.nvtphybridge.com/ or https://www.dualcomm.com/collections/ethernet-over-coax/prod...

The primary question is whether you want just the cable or the multi drop bus architecture.

And well, DVB-T2 at UHF frequencies works over RG-58 and the just parallel connected T-pieces just fine, and the fact that whole such system has completely wrong impedance does not seem to matter for ~4 devices.

It has the wrong impedance for MoCA, and any long split-off sections might cause nasty reflections, but other than that, MoCA (possibly through an impedance matching transformer) ought to work fine.