Block Apple update servers on your network if you don’t want people updating. Then setup a policy to alert you if anybody circumvents and updates.
Apple defines what the restriction payloads can do, not Meraki. You should submit this feedback to Apple directly.
You can choose to block iOS update servers as part of iOS configuration payload which is effectively the same thing. Blank blocking iOS updates company wide is not something I'd recommend.
Another thing to note about the software update delay payload: it defers X days from when Apple releases the update to their catalog, it does not defer from the date you push/install the profile.