Toilet paper really is an odd invention. I believe there was a time when it was first marketed that the public thought it vaguely disgusting, and as you suggest, an unnecessary luxury.

One plausible explanation for the surge in toilet paper purchase is that in countries experiencing some form of lock down, more people work at home hence the demand from toilet paper moves from office wholesalers to retail shops and the supply chain takes time to adapt. In the mean time shelves will be empty.

There's a ruined Roman bath house near me situated on the Antonine Wall (Scotland). It has a latrine and evidence suggests they used moss to clean regio affectus.