Valve is, by all accounts, a toxic cesspit that's ruled by various influential cabals. They still have a hierarchy, they just aren't explicit about it.

At least for Medium, "Holacracy" pretty much imploded. Zsppos switched from it to an internal marketplace - which is, oddly, controlled by excecutives.

Flat organizations beyond a certain size either move away from flatness officially, create unacknowledged hierarchy, or fail. I'm really not aware of a single large-scale example that still works.

This is much different for small orgs - there's something about org size that requires some sort of centralization. (I think it's communications overhead + complexity exceeding the limit of what a single person can keep in their mind. I'm not 100% convinced, but these are the pressure points I usually see)

The video game sector is unstable and toxic. IMO, Valve compares favorably to other video game companies as for one thing it’s still in business.

Flat organizations are unusual, but survive about as well as other similar organizations. The difference is when a giant reorganization/buyout etc hits, they more obviously change into something else.