This feature was exclusively part of a joke version and is not currently planned for addition to regular Minecraft. It is documented here on the wiki due to being officially-released content by Mojang Studios, even if not present in a final version.
Packed air can permanently fills the player's breath bar when standing in it.
It can only be placed in closed rooms on the moon otherwise it disappears like water in the nether when placing it in an open space on the moon, anywhere in another dimension or when the closed room is opened.
There is a maximum size of room that can be filled with only one packed air, which is 108 blocks³