Is it appropriate to email a professor about something unrelated to class? I have a professor currently that I have previously had in a class on a different subject.  Would it be appropriate to email them with a link to an article I found that relates to the previous class' subject (but not to the current one)?  There are no longer office hours due to school moving online, but I'm unsure if an email would be considered a waste of time because it would essentially just be sharing an article that she may have already read.
I assume you mean something related to academics rather than a completely random meme/cat video.

If so: yes, it’s totally appropriate. Discussing cool research topics with students, especially when it’s free from the context of grading, is one of the most fun aspects of a professor’s work. I absolutely love it when students email me links like that.

And if the professor thinks it’s a waste of time, they’ll reply with a curt thanks and ignore the link, or simply ignore your email altogether.