Oh, I’m completely with you on this and always prefix my namespaces. Occasionally I will alias long namespaces to short ones, but I never pull identifiers into the global scope and I really dislike when I see code online that does “using namespace” (unless it’s tightly scoped, at least). I’ve been prefixing std:: for years and won’t stop now, I like knowing where an identifier is coming from, which is extra important when you have multiple types of similar containers that you use for different performance characteristics (eg abseil, folly, immer versions of containers vs std containers)
You're free to, but I discourage the habit. It's more verbose to add the namespace:: prefix to symbols, but it sure does make it easier on the devs that have to work with the code later.