If I recall correctly, the handle you deleted stays inactive and is unavailable to new registrants. This is present on Google at least, I assume it's the same elsewhere.
The concept of a handle goes beyond a username. If someone can construct a profile that looks like someone's profile on another site and contains approximately the words in the username, like _username or imusername instead of username, they might be able to impersonate it. In that case it would be good to have an active profile on that platform to counteract it.
