Stevenage is my local station and I've been quite vocal about this issue on here. Apologies in advance for the long post, but this is a summary of the situation as I understand it.
Firstly, why is LNER doing this. No-one really knows 100% but there are three theories:
1. Making the new timetable 'work'
This, at least publically, is what LNER are saying. Saving 45 seconds at each Stevenage call means apparently make the timetable more robust. LNER told the local MP that there is only an average of 15 people alighting from each of their trains at Stevenage - if it is so low then why do they need the restriction?
2. LNER do not want to be involved in Project Oval
LNER has been involved in Project Oval discussions for years so it does seem strange if they do not want to be part of it. Even ore so when they are a nationalised operator with the money all going to the same pot! It's even more strange when you consider that LNER has sold cheaper 'LNER only' fares for years.
LNER will get a share of the ORCATS revenue so they will lose this when their trains go pick-up/set down only.
3. LNER do not want 'Stevenage commuters' packing out their long distance trains
This really isn't a thing. I regularly travel on the 17:33 or 18:33 LNER trains from Kings Cross and they are best described as 'fresh air expresses'. The unreserved coach C generally less than a third full upon leaving Kings Cross (I've been counting for the last few weeks...)
Furthermore, many more people are boarding/alighting at Stevenage to make journeys to/from the north. Everyone boarding northbound or alighting southbound means an empty seat between Stevenage and Kings Cross.
How will it all work?
It seems to me that this simply cannot be effectively enforced.
The ECML through Stevenage is paired by direction. There are two island platforms - platforms 1 and 2 share island at which all southbound trains call. Platforms 3 and 4 ahre the other island platform at which the northbound trains call. LNER has no staff at Stevenage station so it'll be up to GTR's staff to enforce it.
Stevenage has a 'turn up and go' service to London. There are 4 fast Thameslink trains and 2 stoppers an hour to London plus 2 LNER trains in most hours. People simply board the first train that arrives. That includes LNER, especially when Thameslink is delayed. The trains will still call because people from the north will need to alight. These trains are 9-cars long filling the whole length of the platform. Local door operation isn't an option. Even if staff start shouting at customers (is that really the sort of railway we want?) I suspect people will just get on, especially when there are delays. In theory, a ticket check can take place (the reality is they don't given the short time between Stevenage and Kings Cross). A ticket block could be done upon arrival at Kings Cross but that's not going to go down well with people who have travelled long distances just to catch a few errant Stevenage passengers.
Northbound is easier to enforce becasue Stevenage won't be shown as a calling point on the departure board. Of course, those 'in the know' will just get on and in my experience ticket checks between Kings Cross and Stevenage don't happen.
Lumo being pick-up/set down only at Stevenage was mentioned upthread. Their trains are shorter and generally at the extremes of the day so less of an issue. Also, they've never conveyed passengers between Kings Cross and Stevenage unlike LNER who have actively encouranged passengers only to now say they don't want them.
LNER senior management recently held a 'meet the manager' session at Stevenage station alongside the local MP. I went along and voiced my concern about how practical pick-up/set down restrictions were. They told me that it works at Watford Junction and Reading. Watford Junction has a different track layout compared to Stevenage (paired by use rather than direction) which makes enforcement easier. At Reading it's a handful of trains a day.
We should also consider what happens during disruption (today being an excellent example). LNER is needed to move people when Thameslink throw in the towel. I raised this point at the meet the manager session and it seemed to me that it was something they hadn't even considered!
The local MP has been quite vocal about this and I know he has met
@David Horne in person about it. LNER instructing their staff to abide by the restrictions suggests they are concerned about adverse publicity.
I've yet to meet anyone who things this is a good idea....