I was able to make Shift+Space to change input language but then Gnome's default settings app made Space to change input language as well. It is not what I wanted.
I wasn't able to find Shift+Super in Gnome Tweak Tools.
Is there some place where I can manually enter key names for switching input language?
In dconf editor go to this path and undo the setting that sets
<Space>
as a shortcut to switch the input source backward.It is impossible to set
Shift + Super
to switch input language.