Perhaps I am missing something, but it appears to me that other than the much talked about advantage of the "thumb cut" for easing loading of the magazine by stripper clips:

Mauser drawings made before the release of the M98, prove that the "thumb cut" had a function as a vent area for any escaped high-pressure gas entering the LHS lug raceway - remember also that the vents in the bolt body exit into the LH lug raceway. Obviously the M98 was not the first Mauser design to have this feature. The RHS lug raceway is largely blocked by the extractor, so with no vent hole in most M98 receiver rings (eg the Mexican Mauser is an exception) the LHS lug way is the primary escape route for gas and flying debris.

It's true that some Oberndorf made commercial actions had a solid left wall. All those I have seen were made well after Paul Mauser died (perhaps this is not the case as they are pretty thin on the ground and I've only seen a handful). The ones I have handled all were relieved on the upper side of the LHS raceway with a small neat area relieved immediately in front of the rear bridge which can only have served to vent gas.

Without some sort of vent, in the event of a high pressure gas leakage for whatever reason, the M98's "full bolt shroud" would be sorely tested, the final barrier to the shooters head.

I've often wondered which is better. The thumb cut provides a huge area for gas release, but what about the shooters face, assuming a RH shooter using a std RH M98. The solid left wall with the much smaller vent area might appear to be a disadvantage, but the gas and any debris is vented upwards. I suppose Mauser must have tested these things back in the day.