I don’t see any issue with the Knights of Ren not being mentioned in TLJ. There are plenty of things that are giving a passing mention in the SW films that aren’t expanded upon immediately.

Anyway, I’m pretty sure they are mentioned in TLJ - Luke says Ben and a “handful” of his students burned down the temple. I figure that’s them.

Now, would it be weird if they don’t show up at all in the ST? Yeah, it would be, definitely. But would it really be bullshit, and is it really that big an omission that they’re not in TLJ at all? No, of course not. This is another case of fans expecting certain things based on the countless speculative listicles that popped up in 2015 that gave relatively minor aspects of TFA an over enlarged importance. If they factor into the story of IX, great. If they don’t, that’s too bad, but I’m not going to lose any sleep over it.