Before 1.18, the minecraft:seagrass_simple feature file existed and was referenced in ocean biome files to place underground seagrass. In 1.18 and above, it no longer generates. With MC-262339 (my original bug report in which I explain why the feature never generates) being marked as WAI, I'm under the assumption that this feature is supposed to never generate, and thus there is no reason to having placed and configured feature files for it.