They're allowed, but they're discouraged.
The problem with URL shorteners is that they obfuscate the final destination and prone to breaking, causing linkrot.
When you use the original link instead, it's far easier to track down a replacement link should that page you link to die. When you wrap around a URL shortener, it's a guess if it's porn or not and which video you wanted them to watch.
Better edit in the actual URL being used now when we can still see the redirect instead of waiting for when it's gone and having a hole of nothing to deal with.
Err on the side of being plain and direct, which the shorteners are not.
