The answer to this is: yes! Once the race is over, the track is fully open to walk around. There is free access to the old banking, which can be easily reached from the main track. When I went (which was 1988 and 1993, a long time ago!) the track had disintegrated very badly, but it was resurfaced a few years ago. You wouldn’t believe how steep it is, to the point you can’t stand upright on it.
Incidentally, don’t bother buying general admission tickets at Monza, you’ll see nothing. The Italian’s rope off big sections of the general admission enclosure and stay there all day. And it is very hot at that time of year out there, so take plenty of sun cream.
Enjoy!