Surely there is very little that can be done for this? The Elizabeth Line uses GWR Platforms at Paddington, and people can get on those at the unmanned stations past Reading. It's very hard for guards to enforce it on the GWR trains before Reading, as they are so quiet before Reading there is always an argument if they're told they can't have them on the train.
Perhaps this is also the fault of passengers going through gatelines though? I have twice nearly been pushed onto the tracks by crowds at Liverpool Street. Once because the crowd I was in was being held back by staff and pushed in, pushing people in it towards the line, however another time when I was coming into London during the evening peak, and the few people getting off were directed to the edge of the crush by staff, squeezing between the platform edge and crowd, however the crowd again expanded outwards and nearly pushed me infant of a moving train!
Regardless these issues should hopefully calm down when through running starts.