Well, I don’t know about this particular word, but I know that some words have been removed from WaniKani, so it’s possible it was added and then removed at some point…
You may have just assumed it was on WK, I do that all the time when I see a word with kanji I know. I always think I forgot the meaning, but then it turns out to be some new word I didn’t learn on WK at all.
I have a dump of WK’s vocab entries using the API dating from last January, the only change involving 以来 since then is the disappearance of this example sentence for 穴場:
Ever since then I’ve never wanted to have that kind of experience again, or to cause trouble for others, so I’ve stopped going to the little-known place where you can fish for fugu.
There are still 3 other example sentences involving 以来, but if a vocabulary entry ever existed it would been purged before the 28th of January of this year and not using the usual deprecation method which I find very unlikely.
I also had the feeling that it was on wanikani but it could’ve just been seen in context and figuring that the kanjis are taught here it makes sense. Do you happened to use bunpro? It’s taught there.
No, I don’t use bunpro but I have an Anki deck containing 以来 and wanikani has the similar 以降. So no issue with WK, it was all a classic case of my mortal enemy, memory interference.