Surprisingly the most recent Patent (that I've read) relating to POP papers dates from 1956. It has to do with stability and improving keeping properties of POP's which is done using a Sodium Metaborate coating to the back of each sheet, which is in contact with the emulsion surface of the next sheet. An extension to this is to use Sodium Metaborate interleaving impregnated sheets of thin paper.

Kentmere use a thin interleaving sheet but whether theirs were impregnated with something I don't know.

It isn't that many years since Ilford stopped making their own POP paper, it wasn't sold to Photographers, instead it was packaged in education packs for schools for children to explre light sensitivity and make photograms etc.

There is still a need for POP's as it's one of the best methods for getting the best prints from many older pyro developed glass plate, although of course you could use digital I suppose