That’s a static rule that the pilots can (and have to) comply with based on information available to them, though.
I wonder if an updated (international) flight plan is checked for projected congestion levels (based on other flight plans?) at all and can be declined for that reason?
The flightplans get shared and updated with every involved area. Usually those can say no ahead of time and always at the time of airspace entry (entering US airspace for example)
What checks the FAA does to those plans, I doubt we'll ever know
I wonder if an updated (international) flight plan is checked for projected congestion levels (based on other flight plans?) at all and can be declined for that reason?