As I understand it the public-facing rule has always been that you must stick to your itinerary full stop, but the rule as far as the industry was concerned (and thus the one that was applied and enforced in practice) was as
@Tazi Hupefi describes.
Nobody in the industry seemed to think it was a problem that they were selling tickets with less advertised flexibility than the tickets had in reality. The idea that passengers might value the flexibility they had on the connecting legs or that they might need or want to know what the actual rules were (without having to come to an online forum such as this one) seemed not to have occurred to anyone, or at least wasn't regarded as significant. Better to just tell people they can't do stuff seemed to be the attitude.
But of course, people noticed that people who checked tickets on the connecting legs didn't want to look at the reservation coupon (and would sometimes even say things like 'don't worry about that, as long as you've got a ticket') so a lot of people (not just on this forum, but 'normies' as well) became familiar with the flexibility of the actual rules even though they weren't advertised or written down.
The industry then made an unannounced technical change, during lockdown when nobody was travelling, which made thousands of previously unreservable local and regional trains reservable and therefore made the booked itinerary enforceable on the connecting legs of a journey. They didn't reverse that when lockdown ended. And then they wonder why people don't understand the rules, which haven't changed on paper but have changed fundamentally in practice.
Even if passengers were told that they have flexibility on unreservable but not reservable trains (which they're not), they would also need to know, where their itinerary tells them to take a specific train but doesn't give them a seat reservation, whether that is an unreservable service or a reservable service with counted places. Which they're not. The ordinary passenger has bob hope of understanding this.
In south east England there's a further complication where GTR elects not to enforce reservations some of the time (i.e. where they regard their train as a connecting leg rather than the main leg - neither of these terms has a precise definition and the customer isn't told how their booked journey breaks down between the main and the connecting legs).
It's really rather inconvenient. Where I have a connecting leg on a reservable service, if it's a high-frequency service or there's a realistic possibility of making an unofficial connection, I normally end up working out my own split so that I buy a flexible ticket on the connecting leg. Otherwise you can, for example, arrive on time in Leeds from London 9 minutes before a connecting local train is leaving, have to watch it leave and then wait half an hour for your booked connection that meets the MCT. That seems to me to be pretty obviously anti-passenger. I don't agree with the notion that most people just want to be given an itinerary and follow that - if, on the day, an earlier connection is available, people would prefer to take it if allowed to. And people should be told whether they're allowed to or not, rather than have to guess which TOCs and guards will or will not enforce it.
How does the reservations affect if it's valid or not? - sidenote I understand you (used) to be able to travel on any connecting services with an "& connections" ticket if you didn't have a reservation. But you did have to travel on a specific service if you did have a reservation. Is it related to that?
