It seems to be a specialized term referring to the change in focus of blogs [0] and online communities [1] over time. This strikes me as a very specialized concept, rather than a generally-important term in machine learning as a whole.

Edit: To add my perspective, with years of industry experience and graduate-level machine learning coursework, I have never before encountered this term.

[0]: https://link.springer.com/chapter/10.1007/978-3-319-16354-3_...

[1]: http://catb.org/jargon/html/T/topic-drift.html