Spent way too long debugging why S-SPC suddenly stopped working - I figured it was my config or some updated package. It was not - I had also switched from emacs to emacs-wayland on my system (I use Arch btw), turns out pgtk does not support S-SPC as it is used by input methods. I don’t use it (as far as I am aware) - so disabling it made it work again:
(pgtk-use-im-context nil)
I am using it to trigger corfu (and orderless), which turns out after this stopped working has become so essential that I felt like completions were completely broken without it. If you haven’t tried setting this up I highly recommend it.
Anyway, maybe this helps some poor soul with the same problem.
Source: https://lists.gnu.org/archive/html/bug-gnu-emacs/2021-07/msg00071.html