Walls – Removal of a 2×4 with roof truss built on top

You'll need to header off the cut truss. This will transfer and redistribute the load on the cut truss to the adjacent trusses. To do this, you'll need to remove the tub because you'll need the space to work, or you can do this from the room below and cut the ceiling for access. Either way, I would still recommend moving the tub so you don't have to deal with the weight. Support both sides of the truss cut truss from below. For the headers, cut out a section of the sagging truss where the p trap is. Sixteen or so inches should suffice. Cut two 2x12s long enough to run perpendicular to the cut truss between the adjacent trusses, then cut two more for the other end of the sagging truss. Cut four pieces of ¾" plywood as long as the headers, and as wide as the truss, then fasten two to each 2x12 header. So now you have a box around the p trap. If the drain line needs to run through the header, just cut a hole through it to run the pipe. Putting in these headers is structurally sound, and should also satisfy local code. Also, keep in mind that this is not a trivial task, and I wouldn't recommend a casual diyer doing this. I would instead find a local carpenter with experience in this area.

If its parallel to the joist, you can be pretty sure its not holding anything up other than the 2x6. I'd guess the purpose of the 2x6 is probably there for the edges of the ceiling to be nailed to.