I put my warp wither down at y 4 I believe, and you can't hear it. The y level of the warp is y 70/71 universally out 50 blocks in each direction. I think an effective way to have withers at warps is to have them at bedrock level, steady, with a 3x4 space to fly around in so they don't suffocate.
They still agro but the sound can not be heard at all. I don't support this decision, but I believe a rule should be added to instead make it so if you put a wither, put it as far down as you can.
It should also be noted that the bossbar will not be affected by height, only the X and Z coordinates. So if you have a wither at y 100 and you're at y 800, the bossbar should still be visible.