Are you in en_US? The sequence you mentioned should definitely work in that locale. Here’s a more authoritative link than the manually-updated wiki page: https://gitlab.freedesktop.org/xorg/lib/libx11/-/blob/master...

If you’re not in en_US you can find other locales and their Compose sequences in the nls directory.

what do you get using comma?

The same thing, ç

edit: hold on, that's only in Firefox. Anywhere else, comma+c does ç and apostrophe+c does ć