[2019] Do I have to keep The Closed Test version of this game 20GB in my library or I can delete it?

Also - if you're looking to make some space on your drive, you could also clear your 'console logs' and 'crash dumps' directories which are located here:

1. Press the Windows key + R
2. Enter %appdata% within the search input and select 'OK'
3. Navigate to AppData\Roaming\Fatshark\Vermintide 2