P:(Masking) Shortcut Key for Select Subject

Good idea. While waiting for Adobe to implement it, you can do this with a simple script:

1. Do Preferences > Presets > Show All Other Lightroom Presets, which will open File Explorer / Finder on the "Lightroom" preferences folder.

2. Create a subfolder "Scripts" in the "Lightroom" folder if it doesn't already exist.

3. Using Notepad / Texteditor, copy/paste the following two lines into a new file and save it as "selectsubject.lua" in the "Scripts" folder:

local LrDevelopController = import "LrDevelopController"
LrDevelopController.createNewMask ("aiSelection", "subject")

4. Restart LR. 

5. You can now create a new Subject mask by clicking the Scripts menu and then "selectsubject". (The Scripts menu is a scroll icon on Mac.)

6. On Mac, assign a shortcut key to "selectsubject" using System Preferences. On Windows, use the free Autohotkey utility to assign a shortcut to "selectsubject".