The variables in the aircraft.cfg have no affect on the visibility of the registration. Please stop telling people to do this. It used to work in FSX, yes, but never in MSFS.
Using a space character for the registration defeats the purpose of having a registration. Usually people want ATC to know what their registration is so they can recognize when they are being called.
The only way to fix this is to properly edit the panel.cfg file to reduce the size of the registration text to zero. In the past, you have also often needed a copy of the panel.xml file in the liveries panel directory. The best way to do this is to use the following code in the panel.xml file. That way, it uses the panel.xml file of the default plane, and you don’t have to worry about keeping it up to date.
<Include RelativeFile="../../Carenado_M20R_Ovation/panel/panel.xml"/>
This example is for the Carenado M20 (obviously). For other planes, you’ll need to replace the “Carenado_M20R_Ovation” with the name of the directory of the default plane the livery is for that you’ll find inside its SimObjects\Airplanes directory, as was done for the M20 example above.
You can control many aspects of the Registration (except the size of the decal used to create it) by changing the following code in the panel.cfg file
[VPainting13]
size_mm = 1, 1
texture = $RegistrationNumber05
location = exterior
painting00=CustomRegistration/CustomRegistration.html?font_style=l3&font_color=0x360000&font_scale=1.15, 0, -50, 1600, 620
Notice that the size_mm = 1,1 instead of whatever it may have been. By reducing it to 1 millimeter (or 0), the registration text will effectively disappear and you can paint over it. You can also play with the scale or the color of the text with this.
If there are any instruments other than default instruments called out in the panel.cfg file, you’ll need to change the path for them to point to the panel directory of the plane the livery is for.