Please explain your feature request to the best of your abilities:

At present, if a homeblock is set and then covered deeply underground, when players /t spawn there (I assume also /t outpost.), they suffocate with no way to escape. This can be abused by players.

Proposed solution: If the spawn location is buried in non-breatheable blocks, spawn at the closest Y-level at that X/Z coordinate with two breatheable blocks (or the world height, if no double breathables are found).

Seemingly good but actually fraught solution: Preventing players from placing things in the two voxels above the homeblock spawn surface. The problem with this solution, is that it prevents putting buttons and pressure plates and such. I know pressure plates can be used for evil; but in my own town, the pressure plate on the spawn surface actually plays the town anthem for spawner-inners. So, imho preventing the placement of carpet and redstone and whatever at the homeblock would be throwing out the baby with the bathwater. (This is also why I specified "breathable" blocks instead of just "air" blocks above.)

I'd have to see if the location can be altered when we teleport using PaperLib. This one should be a rule in your server that is heavily punished.

This one should be a rule in your server that is heavily punished.

I definitely agree there, and it will be so.