Threatened to be fired or have pay cut. Should I still sign the document? Is this legal?

tldr;
Boss doesn’t think they have given me enough tasks at work.
Says it’s their own fault.
I’ve been filling up extra time with auxiliary tasks.
Still threatens to fire me over it and to sign document justifying it.
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We had performance meetings at my office. I was told my deliveries are fast and high quality, but they wish they had more tasks for me and that it was their own fault. I said I was filling up extra time by supporting team members (which they already knew.)

I was told to suggest projects for myself using a template. The template was a Performance Improvement Plan (PIP) for a different employee with slow and low-quality deliveries. I said I was happy to come up with more projects for myself but asked if I could submit a Project Proposal-document instead of PIP since those complaints didn’t apply to me. They agreed but still sent me a PIP template that stated:

“Failure to meet the expected performance and quality standards may result in pay cut or termination.”
The complaints for my colleague had been removed though.

I’m supposed to sign that document at the submission date. Don’t know if I should sign since I think this might violate labor laws. Those standards are supposed be defined by myself along with project suggestions covering many details. I’ll be scrambling to submit in time and doubt I’ll get time to do sufficient research so I’m reaching out here. Would you sign it? What would you do? Some might say I should find another job but the market is terrible for mechanical engineers like me, especially since my Japanese isn’t good.

by dendaera