See some discussion in T6878 and Q467.

Until recently, we did not import "raw" refs (refs/custom/whatever) in Diffusion. After T6878 we do, but we don't show them in the UI.

They're "sort of" reachable like this:

https://secure.phabricator.com/diffusion/P/browse/master/;refs/heads/stable

And this probably "should" work:

https://secure.phabricator.com/diffusion/P/browse/refs%252Fheads%252Fstable/

It would be good to have a list of all refs similar to this list of all branches available:

https://secure.phabricator.com/diffusion/P/branches/master/

That should also be linked in the UI somewhere, but probably not shown on the repository homepage (in contrast to tags/branches, which are shown) because there's almost no use case for browsing refs in normal use (all use cases are likely internal/technical stuff). A subtle, somewhat-buried link or button somewhere to get to this view is likely sufficient.