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.

The transfer valve below was packed with JB Weld to negate it's operation.

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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.

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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