A victory for Bayer at the Supreme Court could establish that, because the E.P.A. has reviewed glyphosate and determined it does not require a cancer warning, individual states cannot allow lawsuits that claim the product is mislabeled because it lacks that warning. That could eliminate a substantial portion of Bayer’s liability, said Nora Freeman Engstrom, a professor of law at Stanford Law School. “It would close the books on substantial liability,” she said. “But not all.”