When the "ff-only" merge strategy first became available -- it could also be selected when doing a merge on the website (not as a part of a pull request). As of a few months ago, the "merge strategy" option seems to have disappeared from the merge dialog. You can still select a merge strategy if you are merging a pull request -- but if you just select "Merge" from a branch page -- it is not available.
Bringing back this option would be very helpful -- especially when merging a release candidate into master after a deploy to Production. There is no need for a pull request at this point, and I want to be able to fast-forward merge into master.