Apple - 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.
This is not a standalone answer, but since comments can't contain pictures, I added a new answer showing the picture requested in a comment above.
Here's a picture that shows a magnet in action, fooling the lid sensor into believing the lid is closed while the lid is open. The purple thing is a magnet used to attach a paper to a whiteboard. The paperclip on the screen shows how I found where the lid magnet was. I tried to be careful to not scratch my screen (actually the border of the screen) while moving it around to find the location.