Not at all theoretical, this happens all the time. There are tools like StegoShare: https://en.wikipedia.org/wiki/StegoShare

I googled and one of the top articles was this (I didn't read it): https://www.deccanchronicle.com/technology/in-other-news/070...

I couldn't find the article I had read a few years back, but I remember this sort of thing being used to host content on Facebook, Wikipedia, Reddit, etc, before they cracked down on it.