Created attachment 238321 [details]
shift-space-should-be-allowed-as-a-shortcut-key.png

When trying to set “Shift+Space” as the shortcut key for
“Switch to next input source” or “Switch to previous input source”,
one gets the message:

    “The shortcut "Shift+Space" cannot be used because it will become
     impossible to type using this key. Please try with a key such as
     Control, Alt or Shift at the same time.”

See screen shot.

But “Shift+Space” is usually OK to use and should be allowed here.