Custom domain
A domain you own, pointed at a link shortener, so your short links carry your name instead of the shortener's.
Setting one up is a CNAME record and a certificate, which usually takes minutes. A subdomain of a domain you already own, such as go.example.com, works exactly as well as a short new domain and costs nothing extra.
It matters more than it sounds. A short link hides its destination, so the domain is the only signal the reader gets about who is asking. It is also what keeps generic shortener domains, which are heavily used for phishing, from getting your links filtered as a category rather than judged one at a time.
The part people notice last is ownership. If the shortener closes, links on its domain close with it. Links on your domain are a mapping you hold and a name you renew, so you can point them somewhere else and everything already published keeps working.