I think that hiding the branch is not the right approach. The fact that the original developer stopped working on it does not imply that the branch is useless, and we should err on the side of showing things that may be useful.
However, we should prominently display the abandoned status. We might also want to reduce the visibility of the merge proposal, or hide it entirely. It implies that the branch is in development, which is at odds with the branch's status. (We should still allow traversal to the merge proposal from the branch page itself.)
If there were no merge proposal, the Abandoned status would be prominent, so as a first cut, I suggest that we ignore the merge proposals of abandoned branches in this display.
I think that hiding the branch is not the right approach. The fact that the original developer stopped working on it does not imply that the branch is useless, and we should err on the side of showing things that may be useful.
However, we should prominently display the abandoned status. We might also want to reduce the visibility of the merge proposal, or hide it entirely. It implies that the branch is in development, which is at odds with the branch's status. (We should still allow traversal to the merge proposal from the branch page itself.)
If there were no merge proposal, the Abandoned status would be prominent, so as a first cut, I suggest that we ignore the merge proposals of abandoned branches in this display.