Google Product Forums
Re: Username is taken but it is deactivated, how can I use it
The Lady From Uncle
Well YouTube didn't allow it and then Google thought it would be a good idea to allow users to reopen their own channels. Until they recently decided to not permit it again.
Either way- it has never been possible for usernames to be recycled due to the amount of data associated with it which has legal implications, like email addresses often do.