The modifications can be as simple as internal summing of connections, or as complex as reorienting the pair of existing stereo coils. With either (or both) the cartridge still responds to and generates a signal from vertical content which can potentially affect what is heard by the listener.
as for current mono options, I agree that the summing will pick up and attempt to offset any vertical content but if the coils are oriented so they are in the lateral and vertical plane and only the lateral ones are used then I do not see any pickup of the vertical info. The one big difference found in "vintage" mono pickups is there is little vertical compliance which is the origin of the rumors that playing a stereo record with a mono cartridge will surely destroy it. I am not aware of any recent manufacture cartridges that fall into the "lateral compliance only" category.
dave