Oh, it was before 1990. I was a teenager in the 80s and there was a lot of ninjas in movies, cartoons and graphic novels by 1987, I'd say, with it all starting before then.

I ended up watching the movie after seeing this comment.

They never left, they just got better at Ninja-ing so you aren't noticing them. https://www.youtube.com/watch?v=WtR2m20C2YM

In Denmark we have a rather popular trilogy of books/movies called Ternet Ninja which is about the vengeful spirit of a samurai who gets trapped in a little ninja doll and teams up with an awkward teen boy in a boring Danish neighborhood where everybody is a bit crazy.

The trilogy, outside of just being a lot of goofy fun, is actually also a bit of social critique of child labor in third world countries and how we in the west benefit from children's suffering.

The man behind Ternet Ninja is a Danish comic who's been making children's entertainment for years and has this extended universe where Ternet Ninja fits into. The topic of child labor and western exploitation of children is a reoccurring theme in a few of his storylines.