Readme.io and Paligo both support a very simple tool for creating new versions of the documentation using branching. This allows the writers to work on a new version but to keep the previous ones available for customers to view rather than override or clone the page.

Considering Readme.io and Paligo have had this solution in place for many years now, it's obviously not something very cutting edge or difficult to implement.

I considered using your forking solution (suggested by Eddy) but doesn't do the work and simply adds confusion to the writer's workflow.

Any of your customers who have both on-prem and SaaS solutions need to have doc versions, and you're not addressing this important issue.