In Minecraft 1.20+ the best layer is between Y -59 and -53 for diamond ore.
Use a pickaxe with Fortune III to maximize drops when mining veins.
Strip mining at Y -58 is still the most efficient, especially with Efficiency gear.
Avoid mining in deep dark biomes unless you’re ready to deal with wardens.
Ancient cities occasionally contain enchanted diamond gear in loot chests.
Deepslate diamond ore is harder to mine and slower bring Efficiency V or Haste.
Use chunk borders to mine efficiently some players swear by L-shaped tunnels.
Consider bringing a silk touch pickaxe to move and store ores for Fortune mining later.
X-ray texture packs are considered cheating, but some still use them on private worlds.