LA small block,Do you put an oil passage in your head stud?

How's this for engineering:
A .510 stud is 25% larger than than a .440 bolt.
A .07"x.07" slot would reduce the cross sectional area of a bolt shank by about 2.5%. Both shear and tensile strength vary linearly with area.

If anyone is torquing within 2% of failure, they've got other issues.

Losing 2.5% on just one fastener would be fine. Even if the slot caused a stress riser. A good deburr and edge break would help eliminate most stress concentration cause by the slot anyway. Doing the same with a flat, a groove, or any other means which minimizes change in cross section would be equally fine too.

Any interruption of the material grain would be inconsequential, as those factors have a greater impact on fatigue life and crack propagation and not ultimate strength. Studs also aren't TTY and so aren't being plastically deformed. Plus, the grain direction is along the axis of the bolt and so is the modification, so the impact would be minimal.