Paper rawhide is a good analogy Pat.
You can still buy fiber. Vulcanised fiber/fibre is used in the electrical industries, also gaskets, and linings for knife handles.
Most of it is small sheets but it is available in rolls long enough to back a full length bow in one piece.
I haven't tried any yet. I definitely reckon it would would be worth a try.
Strong enough, yet not as strong as fibreglass which tends to cause more set if used on a wood bow that doesn't also have a glassed belly. Whatever the reason was, set or ease of manufacture this style of backing did not last long before ground and tapered laminations with a back and belly of fg took over.