A user pointed out to me today that she was at some point in the past able to select AirDrop from the sharing menu, but that the option is no longer there on her computer. I looked at the Extensions preference pane, at the Share Menu section, and the only options listed there are Reminders, Messages, Notes, and OneNote.
I found the following technote which seemed to describe the problem:
https://support.apple.com/en-us/HT203129
However, following the instruction listed there did not seem to produce any changes.
To my knowledge, this isn't anything I intentionally did.
Any ideas?