There is a popular myth among the Clemson and UofSC fan bases that the two teams are required by law to play one another every season. That is in fact not the case.
Back in 2012, a state representative from Chapin, S.C. feared the annual rivalry game was in trouble due to all the conference realignment that was taking place across the country. He attempted to introduce legislation that required the game be played every season, but it never passed.
Back in the 1950’s, there was a resolution passed that mandated the teams play after Clemson was put on probation. The probation prevented them from playing teams from the Southern Conference, but that mandate is no longer in place.
While we are on the subject of laws and who Clemson is required to play, there is also no law on the books requiring the Tigers and Gamecocks play an in-state FCS school once every season. That is another popular myth that makes the rounds from time to time, and is easily disproven just by taking a look at the South Carolina schedule from every season over the past decade.