It is not that something has changed, just that you have come across a case where your mental model of what a shortcut in Denemo is has gone wrong.
Ctrl+Shift+s is a one-key shortcut in Denemo - you must keep the control key and the shift key held down while you press the "s" key. There is no "quick sequence" involved you can press them as quickly or slowly as you like but you must keep the Ctrl and Shift keys down while you press the "s" key.

What is perhaps a little confusing is that Denemo refers to the
Ctrl+Shft+s combination as Ctrl+S (that is it denotes the Shift+s as "S", an upper-case "s"). This is because it can also be obtained by putting the CapsLock on and pressing and holding Ctrl while pressing "s".

You may not have noticed this before because few standard shortcuts use ctrl with upper case letters - Ctrl+S (that is Ctrl+Shft+s) is one of them, used for "Save As.." but you can lose that one if you never use it. (In fact, I think this may be the only one).

This convention for referring to shortcuts also applies to the combination of Shift plus a letter: if you set Shift+k you will see it is referred to as "K".

HTH
Richard