"to Shanghai" refers to the historical practice of literally kidnapping someone to force them into working as crew on a ship, a practice that was allegedly common in the city of Shanghai. Nowadays, it's colloquially used when unwillingly pressed into service, as in "I was Shanghaied into helping arrange Emily's surprise party".
Considering the practice was usually done by European ship owners and crews, and not by the people of Shanghai, would the term be considered politically incorrect now?
Some things about "Shanghai" most people will nowadays agree on.
would the term be considered politically incorrect now?
Depends so heavily on who you're talking to that I doubt any consensus will be reached. I'd guess that this is the kind of issue where maybe 10% of the developed world's English-speaking population cares enough to be deeply and vociferously offended immediately and use your use of the term as an excuse to get in a long, drawn-out argument.
Another 10% would think it's a bit off, but they'd say nothing.
And the other 80% simply would not care.
Would I use the phrase in an official document? No. Partly because it's vernacular. Would I use the phrase in an e-mail to someone I don't know well? Depends on how much I care about whether they might get mad at me. Would I use the phrase in a verbal conversation in person? Certainly, although given how specific the use-case is, I'd still use it infrequently.
At the end of the day it is based on a thing that did happen and that does materially relate to the place being discussed. Like referring to arson as "Detroit Halloween celebrations". Would the people of Detroit like that description? Maybe not. But it's an evidence-based description.
"Shanghai'ed" and "S/siamese" are both certainly offensive. Better terms for shanghai'ed would be be kidnapped, enslaved, involuntary servitude or trafficked. Better terms for siamese is conjoined (twins); valves, torture or to death I'm not familiar with those colloquialisms, but there has to be alternatives.
