Like many others I use Leo's BU836 joystick controller.
On this, I have connedted 11 encoders and then ran out of digital inputs when I had to mount all my push buttons.

Then I got the idea to do this small print.

It gives 5 ​​extra digital inputs per analog axis.
Be aware that it only works with push buttons and have to be ajusted in FSUIPC.
Pressing multiple buttons at once will cause unintentional input, and can activate some of the other buttons.

The switch AJUST you use when you program the function in FSUIPC.

Go into the "Axis Assignment" press on selected pushbutton (PB 1-5) set AJUST switch to high.

Press FROM in FSUIPC then set AJUST switch to low. Press TO in FSUIPC. Now there is created an area in which the pushbutton work, give your button a command in FSUIPC.

When all buttons are ajusted in FSUIPC set AJUST switch back to NORM.

It seems that there is a fairly large distance between the 5 pushbutton switches offset, so it's probably possible to add more buttons on the same axis