The restrictions started in 2016 according to the post I linked to earlier.
Yes, the restrictions started in 2016 - specifically, there was an RMT strike (as part of the dispute about guards becoming On-Board Supervisors) that affected Southern on Saturday 5th November 2016. As a result, Southern were unable to run any additional services to Lewes (in part, I think, because they would have needed to use the 313s to provide the additional trains, which required guards to operate, unlike the 377s).
As a result of that, Sussex Police requested the closure of Lewes and surrounding stations. I believe the logic was similar to that as to why Brighton was not served at all over Pride weekend in 2023: if Southern don't have enough capacity available to get people home, they shouldn't risk stranding people there and therefore shouldn't run any trains there at all. Whether or not you agree with that argument is another matter, but with a strike on and a limited service running, it made sense in 2016.
The trouble is, for Lewes Bonfire in 2016, it worked
too well; rather than an event that was, in some ways, bordering on unsafe with the sheer volume of crowds (especially from Brighton), the station closures meant that it became a bit more of a local event for Lewes that was more manageable (notwithstanding the issues of trying to get people home at the end of the night). Indeed,
this thread from 2016 specifically mentions that authorities had been looking for ways to reduce attendance for years.
As a result, every year since then (excluding 2020 which had no bonfires), the same closures have been in effect on Bonfire Night, despite the original justification no longer applying.