One way to tell if the panel will accept ctl breakers on sq d, is the panel sch on the door. If it has a dotted or thin line in the middle of the block, it will accept ctl breakers. And sometimes the panel number is an indication, ie 10-20 means 10 reg or 10 ctl. I agree there are many non ctl btrakers used where not allowed
At one time yes. Eventually it got to where one cover sometimes worked on more than one loadcenter, and still is that way today. You can get a 20 circuit loadcenter that is using same cabinet and cover as a 24 circuit version, and even within those parameters one may be rated for tandems in every space, limited number of spaces, or not rated for any tandems at all.
Since they took out the 42 circuit rules, you are seeing more panels that are rated for tandems in every space. But with the need for AFCI's in most things in a dwelling you still are not seeing a panel on new installs that is full of tandem breakers.
This really was a big deal for mobile and manufactured homes at one time, less space and maybe even less weight were big factors in decision to use tandems and quad breakers for those applications, AFCI's took that away at least to some extent.