In 2010 I bought (among other things) from Berlin (e-Bay) some really old paper very cheaply. East german Orwo - from 1980's! Intended to use it as contact copy paper but amazingly most of it has turned out to be in excellent condition. (Now in deep freeze.) Long ago I used really old Agfa Brovira which also was very useable but a bit softer than new.
Are some long ago forbidden chemicals behind the permanence of these ancient papers..?