Depending where the OP is in the world, this sounds ripe for constructive dismissal, but I'd encourage you to really think about it before going down this route. It will create a massive amount of stress for very little reward, and rightly or wrongly can tarnish you for a very long time if made public (because then everyone knows someone wanted to fire you).

Judging by the OPs one other post on HN that they could give up 20 hours of time a week it would also be difficult to get some people to accept that the OP was fully motivated in what they were doing and working their hardest.

I'm not a fan of burning bridges, BUT: I'm no lawyer, but if your pay cut is immediate I cannot see why you can't also leave immediately. If you are in a place with a buoyant job market the most satisfying thing to do might be to just not go back to work and get another job (assuming you can afford to survive the time that might take). Even if your employer does want you out, I bet they aren't prepared for you to leave immediately, a little bit of payback that may cause them a lot of stress and not much for you (although a bit like constructive dismissal, doesn't do your reputation a lot to good).