Using phabricator at Mozilla for a while, I assumed I was missing something but have lots of reports of other users not being able to find this either.
I very regularly want to share a link to a specific part of a diff while discussing the change via chat or some other forum
for example, there are TODO comments I would like to link to.
The workarounds I have seen / use are to inspect the code, find that the code is marked up with tr id="" and link to that manually, or make an inline comment and link to that. On most code review tools I can remember the line numbers are links that when clicked on update the url which can then be shared.