There is no direct equivalent. The closest social analog is the coffeeshop/bar, but that doesn't quite fit technically. The closer technical fits don't align socially.

Your local telephone pole

The people who installed that pole do not actually want you to post things on it.

Shouldn’t have chosen a material so amenable to staples at eye level then with no removal mechanism save for the work of the elements.

Literally anywhere with a flyer stuck on the wall. Or graffiti. Is the railroad liable for graffiti spray painted on trains? (Actually asking)

In all these cases, the person who adds "content" does so without permission from the owner of the thing to which it is added.

A library, record store, or craft markets are the only physical analogues I can think of.

Not everything has to be a business model

the editorial section of the newspaper?

All curated by the newspaper itself. The whole point of Sec 230 is that the host does not need to curate.