>>280114812it's purposefully never addressed, but the anime made even less of a big deal out of it. in the manga, D checks Sousei's pulse after stabbing him, making sure that he's actually dead. in the anime, he just looks at him and starts running away happily.
for spoilers about what actually happened, THAT Red did die, but similar to how the IRA were housing clones of Suzukiri to make divine tool 2.0s, Yellow (Shinya) houses infinite clones of Red so that if anything happens (a spergout, a death out of the view of the general public, just for fun, etc.), Shinya can replace him at any time. only the most recent Sousei clone in the manga is somewhat relaxed, because finally Shinya folds and decides that no matter how strong Sousei's inner rage makes him (also addressed in further detail later on), cleaning up after his messes just isn't worth the trouble anymore.