Hi nick.kiey,
As you have mentioned, older files will provide this option in R21. If you load older files into R21, than you will see a Migrate button in the Caps Tab (Attribute Manager), that. This indicates that the options are given (for now) to have that functionality with older project files., or to update older files.
Since the new Cap functionality has cleaned up with a lot of the problems of the older version, the separation is not part of it.
You can, however, set up an Alpha material with a specific selection to visually exclude the caps. Not ideal (it requires all other materials for this object to be specified with Selections as well), but I have not found another option. (Besides making it editable and delete the N-Gon cap).
Perhaps voice you wish here (Suggestion)
https://www.maxon.net/en/support-center
All the best