Compass is a very small use item for me.
Marsupial and binos are a huge must have for me so I deal with the inconvenience of removing the harness to check a compass.
Yes. I measured it once, and I believe once you get with ~12” of the harness the compass becomes off.
Edit. I looked up my old post:
I did play with a compass while wearing the Marsupial. If facing west, the compass is roughly 20deg off if held 12” from the front of the harness. By 6” the needle is facing the magnet. In this case 90deg off.
If facing roughly North, it swings it 20-30deg off by about 9”, and the full 180deg off by 6”.
Yes it will mess up a compass, this is not an irreversible issue though. If you get your compass far enough away it will continue to work accurately. Magnets will NOT mess up a GPS device, phone or portable.