Reserved usernames fall into several categories: system terms (admin, root, api, www), brand names (google, nike, apple), celebrity handles, government agencies, and security-sensitive words. Each category exists for a specific reason — whether legal protection, user safety, or technical necessity.
The challenge? Every platform handles reservations differently. Twitter reserves different usernames than Instagram. GitHub blocks terms that Discord allows. If you are building an app with user accounts, you need a consistent approach that covers all the edge cases without maintaining a massive spreadsheet.
Most startups learn this lesson the hard way. Someone registers @support before you do, and suddenly your users are getting scammed. Or a bot grabs @admin and starts phishing your employees. The cost of not reserving usernames upfront is almost always higher than doing it right from day one.