Yes, 4K120Hz! My TV could do 144Hz but i couldn't select it so 4K120 seems to be the limit.

Try for yourself. I get 4k120Hz when connecting my laptop directly via HDMI.

Yeah I have tried it for myself. I am limited to 4K60 when using the HDMI 2.0 port on either my M1 Mac mini or M1 Pro MacBook Pro and LG B2 TV. I do get 4K120 with VRR with newer Macs with HDMI 2.1 as well as my Xbox Series X. It has been my understanding that 4K120 with HDR and VRR requires HDMI 2.1, which is why those HDMI 2.0 limited systems don’t work. Not having a Steam Machine myself, I would assume its HDMI 2.0 port would be similarly limited.

Edit: I should add, I do get 4K120 VRR and HDR on the M1 Macs when connected to a monitor via Thunderbolt or Thunderbolt to DisplayPort adapter, and I would expect a Steam Machine to be similar using DisplayPort, but my TV only has HDMI input and so can’t work in this mode (and a Thunderbolt to HDMI adapter doesn’t work either).

I get 4k120Hz when connecting the output from a HDMI 2.0 port on my laptop to the HDMI 2.1 port on my TV (Sony TV, it has 4 HDMI ports, but for some reason only ports 3 and 4 support HDMI 2.1, 120Hz, VRR).