No idea if this is a new improvement but just started looking at the current iteration of Scratch Desktop after realising Scratch 2 was a bit old and needed Air! Anyway, after some of the comments on here and looking at the uninstall string in the registry I discovered that, for version 3.6.0:
"Scratch Desktop Setup 3.6.0.exe" /allusers /S
does what we all long for!
HTH
Martin