How do I Make a system that makes blocks have no gravity unless it has no supports?

Set the blocks to be immovable. Then test if it has supports underneath, if not, set it to moveable.

Wouldn't that make it only up and down? If i applied that system In the first picture, only the left side pillar would stay up, the horizontal part would fall

One approach to see if a structure is on the ground is to start by marking the blocks touching the ground, then mark the blocks touching those, and so on. In the end you will then know which blocks are supported from the ground or not. It’s kind of like a flood fill. Search for posts by me I think I made a digging example that made unsupported blocks crumble.

Making those objects not supported be movable is then an easy solution to have things crumble. To have them fall in chunks of connected blocks is harder. You can do more flood fills to find all the individual chunks or blocks as a first step. But the second step of connecting them together with physics isn’t really possible with the physics behavior. Best you can do is with joints but that can’t make the shapes rigid.

To have rigid shapes the only option I can think of is to not use the physics behavior and either roll your own solution or attempt using a JavaScript library to do the physics I suppose.