IfYouHappen, I rather suspect that the old-style crappy sanitary towels, consisting of basically fluffed loo roll in a paper tea-bag case and slung on a support belt, would have flushed quite well (they certainly disintegrated if, erm, challenged too much).
I can remember the dire embarrassment of First Year Biology, in which we were all marched to the school Bunny incinerators (no rabbits were harmed in the making of this lesson) and told we would need to rip our towels, empty out the insides and then put the outsides in the incinerator.
Fortunately, I was a late bloomer, and technology had moved on before I needed to burn any Bunnies.