Is it advisable to use a URL link shortening service when writing an academic article?
You should never offer a link shortener as the only option in an academic paper, for two reasons:


It's adding another point of failure: if the shortening service is down, then the link cannot be followed.  This is a particular worry over time, since the service may go out of business.
One of the big reasons why link shorteners are so popular is that they keep track of usage statistics.  I'd be offended if I thought an author was using this to monitor when the link was followed, where the people following it were located, etc.


So if you offer a shortened URL, it should only be in addition to the real URL, not in place of it.  However, I'd tend to avoid even that.  It doesn't look professional to me, and I don't think there's much savings for the reader.  (Online papers should have clickable URLs, or at least ones that can be copied and pasted, so this only arises for someone who has a printed copy but no online copy.  That can happen, but it's hardly a major issue.)