It's not clear to me why a +12V supply would be an advantage? Did 1974 have their equivalent of ATX supplies so that meant one less rail the PCB needed to generate compared to silicon-transistor designs?
And there is some interesting cut in the ceramic that is exposing the lead frame on the left side of the package in the picture that is showing where the -2.5V bias note was written. Do you know what that is?
> First, silicon-gate transistors were smaller, faster, and more reliable.
> Second, silicon-gate chips had a layer of polysilicon wiring in addition to the metal wiring; this made chip layouts about twice as dense.8
> Third, metal-gate circuitry required an additional +12 V power supply.
This implies silicon-gate chips DO NOT require the additional power supply.
For your other question, the visible metal trace on top of the IC package is a common thing with ceramic packages. I think the purpose was to ground the metal lid of the IC to protect against noise, static, etc.
If the metal strip is on the top of the ceramic and grounded it makes sense that it would be for ESD protection. Especially as they likely did hand assembly of the PCBs.
any relevancy in modern technology?
It was a cheap to manufacture chip with extremely limited capabilities. We certainly still have those, but the art has advanced to where the "extremely limited" part is not as limiting as it once was.