This is a strategy 100% certainly designed to make it hard for you to get another job. Just leave. What do you have to lose - your job? It may impact your referees but explain that to the next employer, they should understand that you are the victim of a predatory employer.
The Increase in Notice period was coupled with an compensation review given a few months back. They Informed us that they were changing the policy, No signatures at that time.. But I guess the documents signed during joining the company would contain clauses to justify the interim policy changes.
