'Old' cameras used mechanical switches for flash sync circuit completion to fire the flash. These had no problems of voltage sensitivity, unlike the early EOS and other electronic sync circuitry cameras of the late 80's thru early 2000's. If the flash itself has a PC type sync cable capability (not merely hot foot connection) the 'old' camera will properly trigger the flash unit. Hotshoe-to-PC adapter units would work fine for flashes not capable of direct cord connection, as only the central large contact and the edge contacts are used for flash trigger.

As for newer flash units not getting ISO or FL information signal from the camera via PC cord, the flash will merely output at full power all the time (unless you have set the flash for Manual fractional power), and the FL setting will remain at a fixed FL, so be sure to manually set the FL setting to something suitable to cover the FL used on your camera.