GarageBand 10.1.1 Removed MainStage Instruments
To restore all your MainStage3 instruments do this trick which will add new root folder "User patches" in the Instrument selector:


Quit Garageband
Open Terminal
Type these three commands (all are one lines):


mkdir "$HOME/Music/Audio Music Apps/Patches/Instrument"
ln -s "/Applications/MainStage 3.app/Contents/Resources/Patches/Instrument" "$HOME/Music/Audio Music Apps/Patches/Instrument/Mainstage"
exit

Close Terminal and start GarageBand
Locate Mainstage3 instruments in the "User patches" area or use search box


Downside is some patches are now showed twice, but this does not mean your instruments occupy more space or disk or anything. In the steps above, you actually made a symlink from user patch directory into MainStage3 program patch directory, so GarageBand can always access it. This will alway work not matter how many times you update any of the two programs.

Extra trick: You can edit any patch and save it into Patches/Instrument directory and it will also show up (only first strip channel). This works with all Logic Pro X instruments, including Alchemy and others!