Seems like the special features of the Moon in 23w13a_or_b (low-gravity, unbreathable air, Earth appearing (large) on horizon, micrometeoroids) were hardcoded on that build's code, and then specified to the moon dimension with the option "effects": "minecraft:the_moon" on data\minecraft\dimension_type\the_moon.json file.
Since this effects aren't available (yet) in mainline versions to players to create custom dimensions with similar features, the only way to implement this is in a datapack is with the help of a mod.