There are many instances of one or more individuals attempting to cross from the East to the West or vice versa near the Pepperfields.
For example, Fenar brings an army of horsemen from the East to the Pepperfields:
And there he negotiates a peace that includes the right for Easterners to harvest peppers in the Pepperfields, and to end the practice of the Marquis of Pepperfields crossing over to the east to get horses:
I think the "passage" referred to in that last quote is the eastern entrance to the Pepperfields, and is probably the same route used by all three of the above:
And this was apparently not a well known route:
Though it can't have been a very well kept secret, since Easterners have apparently been using this route for thousands of years:
Almost 800 years later, Morrolan attempts to travel from the East to the West, but cannot find a way across the mountains.
Either Morrolan was unable to learn about this passage, or the passage was no longer usable.
A little more than 200 years after that, Miklos travels West, taking an easy path along the River of Faerie back to its source at Lake Fenarr, and then down the other side.
Is this the same passage Fenarr used? Miklos travels with a horse (Bolk) part of the way, and the reason given for why Bolk doesn't cross over to the West with him has nothing to do with the route being physically impassable by a horse. I suspect that this path could have been used by horsemen, and might have been the same route used by Fenarr.
However, on his way back a couple of years later, he cannot find this easy path anymore, and is forced to descend a thousand feet down a sheer cliff face:
One knock against this being the traditional Easterner passage is that we are told that "few ever came this way" and apparently no one had used it since Miklos last used it two years before.
Either the Easterners are no longer harvesting peppers, or this is not the passage they use to bring them back to the East.
Around 26 years later, Vlad travels East from the vicinity of the Pepperfields, but appears to travel southeast using "goat trails" across the mountains:
Why did Morrolan, Miklos, and Vlad not use the route that was sufficiently large enough for thousands of horsemen to travel through?
The route taken by Fenar might have collapsed or been otherwise closed, but if so, how are the Easterners transporting their harvests of peppers back to the East?
Or are they? One solution is that the agreement between the Empire and the East is no longer active. Perhaps it fell apart during the Interregnum, for example.
If it's still active, however, then there must be a path by which the harvested peppers can be transported.