S. John Ross created a great resource for helping GMs and those interested in world building based on Medieval demographics come up with something realistic.
Towards the end of October, 2018, he decided to stop hosting the PDF on his own website and released it for free download from any website that wanted to host it. This does not mean it is public domain, merely free. The open license for the material is on page 6 of the PDF.
There are two websites that created generators based on his work:
I am now hosting this PDF on this page, and will continue to do so as long as I maintain this website.
You can download the PDF here: Medieval Demographics Made Easy.