Can I turn off macOS Catalina's internal display without close the lid while using an external monitor?
Yes, it's possible.
Well, I can do it.  MacBook Pro, MacOS 10.11, if that matters.

Just find the spot where the lid sensor is,
(about 1/3 from the bottom on the left edge of this laptop) and
park a magnet on it.   The internal display goes dark, but 
the external still displays, and the keyboard types, and both 
the trackpad and external pointing devices still work.

It moves the dock onto one of the external screens, of course, and other items shift accordingly, taking a few seconds.