What I expected:
A checklist item should appear like it used to. This has disappeared a few versions ago. I suspect you haven’t noticed it.

Things that might be helpful to know (Agenda version, OS and model, etc):
Latest Agenda, IOS, iPad Pro 12.9” (2018)

In looking for the keyboard type I decided to check the settings. I remember turning predictive text off a long time ago. I noticed that the Shortcuts switch was turned off (under All Keyboards).
I don’t remember ever turning that one off. The good news is that Shift-Cmd-C now works again. Also the shortcut is listed again when you hold down the Command key.
Interestingly, not all shortcuts are removed. There are still quite a few shortcuts available but Shift-Cmd-C is one of the ones affected by the switch. And I had to terminate Agenda and restart it for it to take effect.

Thanks for checking into this for me. Problem solved.