I have never seen anyone use a cypher to encode their spoiler.
People have been using ROT-13 to encode spoiler discussion on usenet since the 1980s; in the 90s, the rec.arts.int-fiction and rec.games.int-fiction usenet groups were major vectors for the proliferation of hobbyist interactive fiction. That practice continued uninterrupted in places like ifMUD and the int-fiction forums.