Does wearing a brace ruin your joints after a long time?

No. Braces are often used to protect joints from excess stress particularly of the muscles around the joints are weak or damaged from disease or trauma. If possible, muscles should be strengthened around joints as they are the primary stabilizers of joints.
Depends on the joint. It depends on the joint and the reason the brace is being worn. The danger is that the muscles supporting the joint will become weak and stiff, so the joint does not have the support it needs.