This diff allows to delete directories in the file browser. The commit https://developer.blender.org/rB8825250f5a85c0c16e74ed144dd2b4a7d752042f did not include this feature.
I thought this feature would require far more changes, but if I'm not mistaken the current diff is all that is needed. BLI_make_file_string seems to cover the need for canonical path creation for directories as well. I've made some basic tests on Linux and Windows and haven't encountered any problems so far.
Bump because of the limited time frame for 2.81 otherwise it'll be in 2.82. Technically it's not a bug and more of an absent feature, but it would be weird to leave that functionality out.
Tested this a bit ago and it worked fine. I could spend some time to do more testing to be sure, but I think it's better to just push this and see if anybody is having any trouble. In worst case we can revert, it's a small code change.