Since the PF10 key became the F10 key it has always activated the File; Edit… menu. I just noticed that this no longer works in Pluma (Ubuntu Mate 16.04 on a Raspberry Pi). I have tried other programs - same thing. This is also true on Ubuntu Mate 18.04 virtual machine running on VMWare - Intel host. Interestingly it still works on the Mate desktop on CentOS 7.5 which is my normal OS (or I would have noticed this long ago 
Is this an Ubuntu “enhancement” or something I need to configure somewhere? I have no idea where to look. This has been the expected behavior of F10 forever.
TIA,
Ken