It was only necessary if the site was configured that way which most typically were. This was to differentiate which server would handle the request, web, file, mail, etc. I’m pretty sure there was no RFC that stated it was a requirement. Even on the early web it wasn’t uncommon to see a www2 or some other numbered prefix for different versions of a site. The reason you see it less now is because like you said browsers will add it, but also because most sites no longer use it as web is typically the assumed default.