There are many debates over the actual function of the transfer valve as well as plugging it.
If you plug it, theoretically no more oil should enter the primary due to sit sumping.
So there is no need to have the transfer function to remove oil from the primary.
However, if the crank seal blows out, you'll end up with a primary full of oil and lower oil flow to the engine due to half of the oil being in the primary.
But that's a separate issue you'd have to fix with or without the transfer valve functioning.

When plugging the transfer valve;
With the dual function of moving excess oil leakage from sit sumping and venting primary compartment air, consensus says to also install a primary vent as well.
See Venting the Primary below.

Options for plugging the vent/hole:

• JB Weld or other epoxy.
  • It's best to remove the transfer valve before plugging it so don't accidently get anything in the crankcase.
    Pop out the screen, (you wont need it) and the hole is behind it. Clean and dry the hole and plug it with JB Weld or something similar. ¹⁾
    The transfer below valve below was packed with JB Weld to negate it's operation.
    
  • You can remove the transfer valve and make a dummy threaded plug to replace it. The threads are 3/4“-16.

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With the transfer valve plugged, the primary can't breath as it can with the transfer valve functioning.
The air path would now be blocked.
Technically, the primary has a vent other than the transfer valve being the clearance in the clutch cable where it enters the primary.
That can also act as a vent as air can enter and escape there (the part where the actual cable goes into the threaded piece that screws into the clutch cable hole). ^{4) 5)}
There is slack (slop) where the cable end is pressure fitted ( by slack adjustment of cable) into the threaded piece.
There is air space open along the cables path through the threaded piece also.
This would give you a vent of sorts as this is not sealed up and pressure would escape at this point.

Drilling a tiny hole in the primary access plug in the top would also be a good idea.

This one was made with a bolt with a hole drilled through the center of it. That hole was counter-bored 7/32” on top and tapped 1/4“-28 to fit a brake bleeder screw.
The bottom half of the bleeder valve was cut off to make it fit the hole better and to hold the battery stand mount down.
A small hose was slid over the bleeder screw routed down.

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