I wouldn't know if 'most of them' don't work, but I know for a fact that I have one (made by Vivanco, using a Prolific chip) that works fine in combination with my IBM 5160. I use it in combination with FastLynx, which still works even under Windows 11. It's the only one I've tried, so I've not encountered any that didn't work.

All of the genuine name-brand ones work fine, stick to older Prolific (the ones Tripp-Lite included with UPSes are great!) or genuine FTDIs.

Current limits usually make this Just Work: clamping diodes sink enough current to drop the voltage from 12V to 5V (or whatever) and the thresholds in the reverse direction are defined so that a 12V circuit still sees 5V as high if there isn't a terrible amount of noise.

Of course, this all depends on sane current limiting behavior from the 12V side. I'm sure there's at least one line driver out there that does a good "gnome swapping terminals on a car battery really fast" impression.

Just to clarify: it is not simply a matter of RS-232 (potentially) going up to 12 V. The low signal level is between -3 V and -12 V. If the adapter complies with RS-232 specifications in this respect, it should work with older hardware. If it doesn't, then there is a good chance it won't.

Unfortunately, many adapters advertise being RS-232 but the specifications go on to say they are TTL. (I'm not sure that's accurate either, but it is the vendor's shorthand for 0 V and 5 V levels.) Some won't say anything at all, which leaves compliance an open question.