Is there a word or phrase meaning to criticise or mock someone without mentioning their name?
On Twitter, people are able to search the site for references to their own name, so people often 'subtweet' other people - tweeting criticism of them without mentioning their name. Google's definition tells me that this only applies to Twitter.
This phenomenon is certainly older than Twitter. The oldest example that I can think of is the (probable) reference to Shakespeare in Robert Greene's 1592 book A Groat's Worth of Wit:
...for there is an upstart Crow, beautified with our feathers, that with his Tygers hart wrapt in a Players hyde, supposes he is as well able to bombast out a blanke verse as the best of you: and being an absolute Johannes fac totum, is in his owne conceit the onely Shake-scene in a countrey.