In case you missed it, all links on Mastodon count as 23 characters towards your limit, no matter how many characters the link really is.
So, you don't need to use link shorteners on Mastodon as they won't actually affect the link's length.
Mastodon does this because it's better for everyone's privacy to avoid link shortener services, it means people can see what they're clicking on, and the link won't stop working if the shortener service shuts down.
More info at:
@feditips something tells me not all clients implement this.
As noted in the link in the original post, the official iPhone/iPad Mastodon app has a bug that means it doesn't implement the 23 character thing.
I'd advise people to switch to a third party app like Toot!, Ice Cubes, Ivory etc as they are better than the official app anyway.