I just wondered if anybody truly knows why and if, how much "Sky-Mob-Farms" got nerfed in the most recent update of the game. I read a lot of YouTube comments saying that ideally the farms should be at the bottom of the world now. I also tried making a mob farm in creative above an ocean biome at height 160Y and I didn't manage to spawn a single mob on my spawning platform in the sky. Are all sky-mob farms broken these days, is my best bet just lighting up all my caves and put it somewhere near sea-level?
Mob farms in the sky didn't really get made worse (or at least not much). It's still useful to build them in the sky if you don't want to light up absolutely all ground, caves and more. The reason is that mobs only spawn within 128 blocks of the player, so you can stand around AFK very high up and only have that farm in the 128 block radius.
What was changed is how the spawning algorithm goes from the bottom to the top. Previously it first tried to spawn mobs within the lowest 16 blocks of height, had a limit and some random chance there, then proceeded with the next 16 blocks and so on. Now it handles every single block of height individually, so every block that your farm is lower makes it more effective and less lag-causing. Previously it didn't matter if you had your spawning platform at height 6 or height 14, now it does.
So in summary: Building your farm slightly lower now helps, before a bit of height difference didn't matter much. Building in the sky is still easier and building very low was always more efficient.