Should the pig that was cast with demons be eaten today as clean meat?
The actual pigs which the demons entered after being cast out of the two Gadarean men threw themselves off of a cliff and drowned around 2,000 years ago, and so it would be impossible to eat them (Matt. 8:28-34).
As for eating pigs in general today, the old covenant of Moses which condemned such was taken out of the way 2,000 years ago when Jesus died on the cross and has been replaced with the new covenant of Christ (Eph. 2:14; Col. 2:14; Heb. 9:15-17). Under the new covenant, the foods God had previously called unclean are now clean (Acts 10:9-16).
During the conference in which it was discussed which aspects of the Law of Moses to bring over to Christianity, the only foods prohibited were anything that was strangled, blood, and anything offered to idols (Acts 15:19-20). Concerning drink, the only thing prohibited is alcoholic beverages (1 Thess. 5:6-8; Eph. 5:18) unless one needs to ingest small amounts for medicinal purposes (1 Tim. 5:23).
As for everything else, Christians were told to not allow anyone to pass judgment on them in questions of food and drink (Col. 2:16).