Without physically seeing it (the images are pretty good, though) I'd say that's a combination of ageing and corrosion. Aluminium alloys can and do age and forgings like these tend to "flake" a bit.

It also looks as if you have the beginnings of the same problem elsewhere on the hub shell.

Finding a replacement shell might be a headache just because the cups, cones & axles were all freely available for a long time so anyone with an odd shell (back in the day, anyway) just made it into a serviceable hub :-)

It depends what you are willing to spend time doing.

You could strip the hub and get it re-anodised and rebuild with good quality spokes and probably 2BA brass washers under the spoke heads, then tie and solder - doing that, load is spread a little by the washers, you've protected the material that is left in the badly affected area and slowed or stopped any corrosion elsewhere.

If you rebuild as it's currently built, x3, the load on the hub flange is sufficiently tangential that I don't think the flange will let go, but if you tie and solder, even if it does, and even given that it's the gear side, the wheel likely won't go disastrously out of true and you won't have the heel of a spoke floating about near the block and chain to potentially cause mischief - the tie / solder should keep that under control.

It's a gamble and it's a lot of faff so it depends how determined you are to save the hub.