Don't want to write this by hand? GodCraft sets minecraft:breathability for you with a switch.Get GodCraftOverview
minecraft:breathability is a block component: it goes in the components object of a custom block. Decides if mobs and players can breathe inside the block. Use "air" for things like plants or open frames, and "solid" to make anyone stuck inside it start to suffocate.
Example
Inside the components object of BP/blocks/<name>.json (values are the defaults or placeholders):
BP/blocks/<name>.json
{
"minecraft:breathability": {}
}