Unless AA changed, main cabin SDFC requires special “E” inventory, which is arguably MORE restrictive than DL’s policy requiring the same inventory as booked, although that ultimately will depend on what fare class you booked into. I believe UA is the same as DL - same inventory.

DL paid FC gets last seat availability so no availability restrictions, although there is a fee unless you are elite. AA has no fee for paid FC but does require same booking code.

That said, AA allows SDC on some international routes (LHR notably) while DL is domestic only; UA is most flexible here. AA doesn’t allow connection point changes, while DL does.