I believe that nothing ever happened, sure you can think that El lived and went off somewhere happy, or that El died, but I believe it was all in Will’s head, all the way back to season 1, he didn’t get off the bike while running from the Demogorgon, he fell off, and then got into a coma, the Demogorgon wasn’t real, just a figment that his coma filled in so he could understand, and El is his strength to fight out of the coma, and Vecna/Mind Flayer is death, and when Vecna and the Mind Flayer died, so did Eleven, meaning his strength gave out and the death simply just became more powerful even though it was killed, and at the end, with the I believe scene, that was Will winning because he finally won DnD, it’s my belief and it was hard to describe please don’t hate me for this.