Given that there appear to be more complaining threads on internet forums, consumer advice sites, press reports & complaints to TF about Northern's 'buy before you board, or risk a possible £80 administrative penalty' policy than the policies of any other TOC over the past couple of years, I doubt that is the case for the vast majority of their customers now.
I disagree as I think it's very regional as Northern have been rolling it out in a very regional fashion.
Those users in Manchester (where Northern have been hot on revenue for a couple of years) are probably well aware of it at this point. Those in West Yorkshire until the last few months were used to the old Pay Train approach even where ticket facilities were provided and will now slowly be changing their behaviour.
However for those in Teesside, Tyneside, South and East Yorkshire I doubt there is much awareness of this as Northern are not enforcing the rules in those areas in the same way they are in the Manchester and West Yorkshire.
Certainly if you asked a train load of people getting of a Northern service at Middlesbrough or Darlington if they knew they should buy a ticket before travel where facilities are available I would gamble the vast majority would say something along the lines of 'can't I just get it from the guard?'.
I think there is a risk in your post of considering Northern to be a homogeneous entity where policies are implemented uniformly across the network. From this passengers perspective Northern in the North East are a very different company from Northern in Greater Manchester (and now West Yorkshire it seems).