Who knows? Who cares? Certainly not Rian Johnson’s TLJ...
You say that like the Knights of Ren were a going concern in TFA, or any of the ancillary material. They were a half-second of footage, and part of one sentence from Snoke. On a per-second basis, they might've gotten more exposure in TLJ depending on how quickly Luke said "Ben left with a handful of my students." Hell, that confirms way more than we already knew for sure; the Knights of Ren weren't a preexisting group Kylo joined, they are Force-users, they're likely personally loyal to him rather than to Snoke or the First Order...
We got exactly as much about them in this movie as we had befroe; "Kylo has a posse. They're not here right now." The Knights of Ren being so underdeveloped as to border on the theoretical is a flaw in The Force Awakens, and unless the plan was for them to come into play later (which, based on how things have developed, may be the case, as we need some secondary bad-guys now that the First Order is down to Hux and Ren), I would argue the failure was not cutting them out of the first movie entirely rather than establishing them without any plans to make use of them dramatically.
Are they still after watching TLJ? I mean Snoke's gone. The only thing that interests me (as a semi-casual viewer) how he made contact with Ben and managed to turn him. But where he comes from? Not sure I care about that anymore as he's a non-entity.
That is the real question. All we of Ben Solo's corruption is him being woken up confused. Luke says Ben was flirting with the Dark Side and then sees a flash-forward when he tries to investigate it (probably the highlights of the Force Awakens, the casual brutality, patricide, and mass-murder), and it's enough to make Luke panic, but it's hard for the viewer to know if that's an understandable action.
As an aside, people's sense of betrayal by Luke is surprising to me, since "Would you kill baby Hitler?" was the hot thought experiment of 2016. Clearly, TLJ asserts the wrongest answer is "Maybe." If Luke hadn't hesitated, that'd be one thing, and if he never considered it, it'd be another, but drawing his weapon
then thinking it over was where it all fell apart. And we certainly know Luke is capable of acting rashly when provoked, and that his great strength as a Jedi wasn't that he never felt temptation, but that he was able to stop himself. He attacked the Emperor in the Throne Room, and then flew into a rage when Vader threatened Leia and came within a hair's breadth of killing him after he was subdued. Exactly the same thing happened here, except his audience wasn't hoping for him to murder someone and, thus, panicked.
Anyway, back to my point, Leia is under the impression Snoke had been grooming Ben for a long time. If she's right, then it gives credence to Luke's suspicions. Another point in Luke's column is Ben's reaction. Rather than running away, or trying to figure out what happened, he burns down his school and kills anyone who doesn't decide to join him on the path to evil (which implies he'd already been cultivating his "Knights"). What if Luke was being mind-controlled, or was some sort of illusion? What if there was a trash-monster in Kylo's tent about to strangle him and drag him into the swamp, and Luke was aiming at that? What if Luke had just watched the Pink Panther movies and decided on a more spontaneous schedule for combat drills? If murdering a student in his sleep who's falling to the dark side isn't the Jedi Way (and, hey, whose to say it's not? In a thousand generations, there have to have been some bad apples before), certainly ransacking a temple and killing all your buddies isn't the proper Jedi response, either.