All links on Mastodon count as 23 characters towards your post’s character limit, no matter how long they really are. Because of this, there is no need to use link shortener services on Mastodon.
Mastodon does this because it’s much better for privacy that links remain in their original form, as link shortener services tend to track the people that click on them. It also means the links will continue to work in the future and aren’t dependent on the existence of the link shortener service.
But I want to track people who click on my links!
You can use link shorteners to track people, but you probably shouldn’t. Also, some people may wonder why you are using link shorteners when all links count for 23 characters anyway.
Why is it 23 characters?
Apparently Twitter’s original built-in link shortener used 23 characters. So when Mastodon removed the need for shorteners, they emphasised this by only counting 23 characters for all links.