Google does not return any results other than this question for the exact phrase "CA Wage Theft Notification". According to this site
The Wage Theft Prevention Act (AB 469) goes into effect on January 1, 2012. The new legislation amends existing laws (Labor Code sections 98, 226, 240, 243, 1174, and 1197.1), and adds new requirements (Labor Code sections 200.5, 1194.3, 1197.2, 1206, and 2810.5) which criminalizes willful violations for non-payment of wages after a court judgment or final administrative order; requires restitution to the employee in addition to a civil penalty for failure to pay minimum wages; requires that specified information be provided to employees at the time of hire
So this sounds like it is a notice given to satisfy the Wage Theft Prevention Act, but it's a bit weird to call it a "Wage Theft Notification".
The above site also says
As of April 11, 2012, the template has been updated to indicate that the Acknowledgment of Receipt section of the template is only optional. Signatures by the employer or employee may provide assurance that the notice was, in fact, provided by the employer and received by the employee.
If you want to be sure your signature isn't taken as consent, put something along the lines of "Signature indicates only receipt and does not constitute acceptance or waiver of any right or claims." right before your signature.
You don't state in the question whether your boss had, in fact, previously discussed this with you, but since you don't say that they had, I assume they hadn't. So you should start out asking what they are talking about and trying to get this clarified. If this can't be resolved in your favor, you'll have to decided whether to accept this or not. If you don't, you should send an email and a written letter stating something along the lines of:
I am currently employed at the salary of $yy,000, as agreed upon in the offer letter given to me on [date]. I have been informed that the company is not willing to continue this employment. It is therefore my understanding that effective 12/17/2020, this company has terminated my employment.
Don't let them frame this as you quitting. You are willing to continue working at your current employment. They are terminating the current terms of the employment and proposing different terms. If continue working after the 17th, you will be agreeing to those other terms.
In either case, you should be considering other employment.