There are Manufacturers that do use constant torque motors in residential furnaces though. Trane S8X2 S8X1 and S8B1, Carrier Comfort and Performance series, York TM9Y, Bryant Preferred and Legacy Line are just a few that use constant torque motors for the blower fan. Basically every Manufacturer offers a lineup that consists of different series that use VCA (Variable Speed Constant Airflow), VCT (Variable Speed Constant Torque) and FCT (Fixed Speed Constant Torque).

That is not entirely true, it depends on the type of fan being used. In a constant torque motor the fan would increase amp draw as it attempts to overcome the higher external static pressure.

Since you say it is a small bathroom originally you would be most likely fine to block the vent off with no real impact. If I had to guess that vent gets maybe 30-50 cfm and is probably less than a 5” duct. Your blower fan will not blow up, your system will adjust itself to the removed duct and the small amount of air will redistribute. Pressure increase would be negligible. While this is not the “proper” way it will work. Best way to block off the run is get as close to the main trunk as you can and use a cap. Second best is to at least get away from the flex duct and on the hard duct again where you can use a cap, I would attempt to avoid just zip tying that off.