There are three important things to note:
- When a work has a DOI, the DOI should always be used instead of a URL (see Section 9.34).
- When a work has no DOI, the URL is not needed if you read the work in print or accessed it via a database (as many students and researchers do for journal articles and book chapters); see Section 9.30 for more details.
- When a reference does include a URL (e.g., typically online-only sources such as tweets), the full URL is usually fine (as shown in many examples in Chapter 10).
Section 9.36 notes that for long and complex URLs, when a shortened version would be more readable and make the reference shorter, a shortened URL is an option. In particular, we envisioned shortened URLs being used by students writing papers for their courses, where the lifespan of the paper is limited. The shortened URL a student creates for a reference in a paper due imminently is very likely to work for the period of time needed by that student and the instructor.
For works that will have a longer lifespan, authors may want to include in the reference the full URL, even if it is a very long URL. Journal editors may also want to stipulate in their manuscript submission guidelines whether they accept short URLs in references.
If you choose to use a short URL, it may be best to stick with well-known services (e.g., bitly, perma.cc) or the short URL that is provided by the site of the long URL (e.g., Amazon provides amzn.com links for its URLs). Those sites are likely to continue functioning for some time, though there are of course no guarantees. Google, for example, did provide shortened URLs, and although they have stopped offering that service, they state that for shortened URLs already created, “all links will continue to redirect to the intended destination.”