there was not any Warning about using new Flash on old Camera´s,
Just because less people are interested in it
Usually people are interested if they can use some of their old gear on a newer camera, and usally some kind of compatibility is provided to ease (and lure) people in upgrading.
Also, older camera have usually less sophisticated electronics (but they could be less protected also).
The T90 was the first TTL camera from Canon, and started to use the four pin flash interface still used - don't know how much the signalling is compatible with later cameras. It used A-TTL, not the E-TTL used now. Again, how much they are compatible at the signal level I do not know.
The A-1 uses an even older non-TTL interface, and has only two pins - it would be interesting to know if Canon when adding pins changed the whole interface or if they are in some ways still compatible.
Anway, I'd be careful not to brick the old camera electronics feeding the wrong signal on the wrong pins... after all the A-1 has no TTL, and all it can do is setting the aperture and sync speed - but it did the other way round, you set the aperture on the flash and it was sent to the camera, not viceversa (flashes then could usually use just a limited set of aperture values for auto operations).