Previously, GitHub only allowed user/project sites to set the repository default branch as their publishing source. This was the main reason why fastpages could not be used to create user/project sites, as its automation deployment uses the gh-pages branch instead.

But now this is no longer the case: you can set the gh-pages branch as the publishing source for your user/organization site.

Thus, fastpages can be used to create user/organization sites.

I have made a Pull Request to adapt the fastpages setup for user/organization sites. Once it will be merged, fastpages support for user/organization sites will be official.