If someone commits to a repository, and assuming I haven't excluded it by setting tracked branches (which comes with the inability to browse the other branches in Diffusion), then the commit will show on the activity feed.
The activity feed offers no option to filter by commit branch, but even if it did, I would still sometimes show commits of more than a single branch in the feed - e.g. master and stable for upstream Phabricator, or master and next/development, or master and selected feature branches that I want to track.
From the feed, I cannot tell the difference between a commit that a coworker pushed to their feature branch, and a commit that landed in master or some other important branch. I don't think it's reasonable to expect users to follow every commit link to figure out its relevance, or even to assume they'll know that it might not have been a master commit. It would be really helpful if the activity feed included the branch in such messages, e.g. like this:
epriestley committed rP08b1a33dc36a: Implement a Git LFS server which supports no operations (on master, authored by epriestley).
I would also expect a new, different feed item if the same commit is pushed to a different branch afterwards:
epriestley committed rP08b1a33dc36a: Implement a Git LFS server which supports no operations (on stable, authored by epriestley).
I can see that this wouldn't benefit your flow, where Differential reviews are used instead of feature branches so feature branch commits never show up in the feed. Your stable branch has only merge commits and those are easy to tell from regular ones, so you get by without branch information.
However, in a flow that uses rebase-and-fast-forward and attempts to avoid merge commits, current Phabricator will make me miss a notification when the same commit was pushed to the master branch that was previously only part of a feature branch. Hence the request to have it show up once for each branch (maybe once for each push?) in the feed, rather than once for each commit.
Whatever Phabricator's solution turns out to be for marrying branches and reviews, it would be helpful to be able to track what's going on on each branch by looking at the activity feed.