Yes it has changed. NJT and LIRR customers have unfettered access to the platforms and tracks. The track assignment is posted, the passengers head to the appropriate platform, the train loads and departs. Just like everywhere else on earth. Only Amtrak treats their passengers like children and then it’s only those that don’t know any better.
New York Penn has always restricted passenger access to the platforms of the intercity trains, at least since I started riding there in 1969 or so. This makes sense because the platforms are so narrow that there's no room for the people getting off and the people boarding to be on the platform at the same time. Back in the PRR/PC/early Amtrak days there were proportionately more NEC trains originating in New York than there are now, but people would crowd the gates until they announced the track and let people down. Of course, people in the know would just go downstairs to the Long Island RR level and board without the crowds.
Philadelphia 30th St. would let you go down on the platform as soon as they knew the track for the train. In any event, even if you came down as part of the cattle line, they would announce the train a good 20-30 minutes before arrival, so no need to be first on the platform. As a kid, I don't remember long lines at the gates.
At Baltimore, it's supposedly "ticketed passengers only," but no one checks you. They announce the trains 10-15 minutes before arrival, and there's never any real lines to get to the platforms and plenty of time to position yourself at your desired spot on the platform. By the way, I used to sometimes board without a ticket if I was running late, and then fire up the app and buy it while on the train before the conductor passed by. You gotta be quick, though, you lose cell service/wifi when the train goes into the tunnels at ach end of the station.
The 1980s renovation of Washington actually had airport-style departure lounges at each gate (which nobody uses for that anymore,) but they usually let people riding the Northeast Direct trains (during the 1990s) wander out to the train without lining up. This all started changing in the early 2000s when they eliminated the unreserved Northeast Regional trains and then had, for a brief time, the gate dragons checking tickets.
It does seem that back in the olden days of circa 1970 - 2000, the trains would consistently be at the platform ready for boarding at the terminals (i.e, WAS, BOS, etc.) a lot earlier than they are now, although when I've used a red cap, I've sometimes been surprised at how early the train is really ready for passengers, as opposed to when they announce the track to let the cattle in.