Changing the "Show the most used apps" to "On" through Registry/GPO

According to my knowledge, there is no such group policy or register setting. The most used app is related to what used recently, and you can only delete the app from the list.

I think that you can disable the most used app list and pin those app you want to the menu.

I will continue research and test, in order to find out if there is any solution or workaround.

Update.

Try the following register.

HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\Advanced
Find the key Start_TrackProgs and double-click on it, if it doesn't exist, just create it.
Name Start_TrackProgs
Type DWORD (32-bit)
Data value 0 (0 uncheck, 1 check)

Re-open the Setting, and the option will be changed.

Hope it can help you!