Prior to Minecraft 1.17, Diamonds were most likely to spawn at Y-levels 5 through 12. Going any further down than that wouldn’t increase the likelihood of you finding diamonds, and in fact, would increase your likelihood of encountering lava.
As of Minecraft 1.18, the game has been updated to change the way that Diamonds spawn. Now, you’ll find diamonds at Y-levels 15 through -63. Y-level -64 is basically just bedrock.