This is an airline requirement. If you do not provide passport info 72 hours before departure, or at time of booking when booking within 72 hours, then the airline reserves the choice to deny you check-in or boarding.

As it's up to the airline, you can expect that the enforcement will probably vary a lot between airlines, airports, and maybe even check-in agents.

There should not be any problem upon arrival. Assuming you were allowed to board, then your passport information was transmitted to CBP on time (unless the airline massively screwed up). The "real" deadline for the airline is something like 30 minutes before airplane doors close.

I don't think late APIS passport information or lack thereof affects getting SSSS, but maybe some airlines could force SSSS on non-compliers if they wanted.