I have a mid 1970’s G&S furnace. The blower often (but not always) gets stuck in the on mode even when heat isn’t being called for.
The only way to stop it is to unplug the unit. Re-connecting even after several hours does not fix the problem.
It does, however, rarely stop in its own.
I tried removing the thermostat (Honeywell T5) from the wall and it kept running.
The filter was recently changed.