It seems to me that this is just the manufacturers CYA.
I have seen the 90 deg requirement on chandeliers where the lamps are 18" or more from the supply conductors, I would think this is more than a sufficient heat sink before the heat could affect the conductors.
In renovating my own house (circa 1939) I left some of the old fixtures as well as installed some antique fixtures that I have rewired and connected to the old wiring and none of these have 90 deg requirements.
I don't know of any prohibition to using old or antique fixtures.
With all that said, I do see this as an issue that an inspector would have to cite if the fixtures instructions included the information/instructions in there listing and labeling