It's mostly obsolete in English, which I think is safe to say and which does not conflict with it being used. For example, I think few people know how to type it into a computer, while everyone who uses a Latin alphabet can type ae.
To be fair I think most English speakers are unfamiliar with how to type most accents. But it appears to me that æ is available as long press "accented character" just like é and ë, also used in English, so they are equally reachable on mobile phone.