How to change Input Source shortcut to work with Shift key? [duplicate]

With macOS' system shortcuts you can't set keyboards shortcuts solely using modifier keys.

You need to specify an additional (non-modifier) key for macOS to accept and save your shortcut.

Sierra does have an extra setting (in system prefs/keyboard/input sources) for switching between certain keyboards via Caps Lock alone, but that does not help solve your problem.

With freshest version (11.4.0 at 13.12.2017) of Karabiner Elements and Cmd + e for previous language source:

You can put this json gist to ~/.config/karabiner/assets/complex_modifications/<filename>.json, add gist rule in Karabiner Elements Preferences -> Complex Modifications -> Rules -> Add rule and Enable Languages -> Command + Shift to Command + e.

This will remap Cmd + Shift to Cmd + e and you can change language source by Cmd + Shift.

Punto Switcher can do it. This utility helps with language auto toggling but you can disable this behaviour and just set desired key binding. I think it doesn't have UI other than Russian, but you will surely be able to configure it intuitively.

I use CommandShift. It's a free and open-source app which "allows you to change input source using Windows-style shortcut (e.g. Command + Shift, Option + Shift, Control + Shift or Fn + Shift)"

Sounds like what you're looking for =)