Shortcut for input source change on MacOS Mojave I have been using Shift+Space for input source change. MacOS Mojave, however, doesn't allow it. The default shortcut is Ctrl+Space and optionally Capslock. I can customize the shortcut except for Shift+Space.

Am I missing something? Please advise.
I think you are mistaken about having used Shift Space for this. Such shortcuts normally include Control or Command.

The Apple defaults for Input Source switching for Mojave, High Sierra, Sierra, and El Capitan are Control Space and Control Option Space. Back in Yosemite it was Command Space and Command Option Space.

Here is another recent query about this.