you have a tapered pipe plug. The female threads are tapered, the male plug threads are tapered. When they are screwed together they lock tightly, and don't usually leak. Because they don't leak, PBblaster, candle, wd-40, teflon, oil, nothing will penetrate into the threads because the threads are designed to seal up tight.
You have a cast STEEL plug, not cast iron. If it were cast iron, a mig would never have really worked. If you've mig welded (or attempted to) cast iron, you'll know what I mean. You might be able to tig it, or better yet braze it, but a mig gun doesn't do squat, real similar to trying to mig weld aluminum with er70s-6 wire.
tapered plugs that screw in tight like that often get seized after they've been in for a length of time. The tighter they are put in initially, the harder they are to get out. My brother is the worst. Every bolt he touches must be thought of as a 2"+ diameter bolt....in other words, crank it down til you feel it start stretching, then a little more & then stop. So the next guy who has to take them out? Good luck!!
yes I've done a bunch of them over the years. Same deal, weld a socket to the square and pray it comes loose. Sometimes they don't and you have to just resort to alternative bolt management, i.e., drill & tap.
it has been said that a 20 minute job is a broken bolt away from being a 2 hour job. Lot of truth to that. Expecially when it comes to turbochargers. The heat affects the material, if the down pipe(s) are bolted rather than clamped via v-bands, almost all of the bolts break off, nearly every time. Doing one now on a Polaris rzr. Yay.