For the great advice on the home made penetrating oil. I just removed a cleanout plug in a cast iron waste line that I don't think has been touched since it was installed over 60 years ago.
ATF, mineral spirits and acetone.
I had to pull pretty hard on a 3/4" ratchet, but when it let go you could see the oil had gotten to the other end of the threads.
BTW, a 1-3/8" 12-pt socket fits over a 1-1/4" square plug with a few light taps.