My experience with headphone outputs on vintage receivers or integrated amps is somewhat limited, though I do have a couple nice little units from the late 70s. Near as I can tell, both have headphone outputs that are just padded down from the main audio path, though I've read about some vintage gear (IIRC some Marantz units) that had proper dedicated headphone amp circuitry. Both my receivers likely have high output impedance since I hear fairly obvious interaction artifacts (e.g. strong hiss, loose bass, a certain lack of overall definition) even with my 250 ohm DT880. I wouldn't use either as my first choice for amping headphones (especially since one is quite sick and is retired from duty indefinitely), but they're not terrible.
Back then headphones were higher impedance on average, so even with higher output impedance a better damping factor was easier to achieve. Also, headphone users were less of a priority back then because, well, headphones were quite a bit worse than they are today and fewer headphone enthusiasts existed. Plus, if you used fancy electrostats or electrets that were actually decent, you had a separate energizer unit that connected to the speaker output, making the quality of the built in headphone output moot. All these points conspire to suggest that vintage audio equipment might have been capable of running headphones well (particularly something like a 600 ohm DT880, either the one from the 80s or the modern one), but I doubt the headphone output was ever much of a priority.