Couldn\'t find anything in the github documentation and also here on SO. But I was wondering if there could be a http://foo.github.com for a private repository
At the moment it is not possible to do that.
However, in the official GitHub public roadmap we can see that this will finally be implemented in Q4 / 2020 for Enterprise Cloud customers.
From the issue description:
Intended Outcome
In cases where organizations want to build an intranet site or publish internal documentation, private pages will allow organization to keep the content private to their organizations.