Federal wage laws state plainly that you must be paid your agreed upon salary for all time worked unless otherwise noted, and we'll get to your specific exception below.
You make this claim, and I just want to make sure first: "Rather than choosing to not pay employees on the scheduled outage days, they have are deducting a small amount from every paycheck throughout the year that will total 5-days-worth of no hours worked." Is this something you agreed to in writing with your employer or did they simply enforce these deductions independently?
The reason for this question, and I'll quickly direct you to this CA excerpt on deductions, there are only so many lawful methods of wage garnishment or deduction an employer can make.
I'll then direct you to this piece from 2009 about furlough, and the co-extensiveness of CA and federal wage law. Since you're a salaried employee, the worry here would be losing that exempt status, you want to read through 7 most importantly:
An employer is not prohibited from prospectively reducing the predetermined salary
amount to be paid regularly to a Part 541 exempt employee during a business or
economic slowdown, provided the change is bona fide and not used as a device to evade
the salary basis requirements. Such a predetermined regular salary reduction, not related
to the quantity or quality of work performed, will not result in loss of the exemption, as
long as the employee still receives on a salary basis at least $455 per week.
This is where it gets tricky. They hopefully had you officially agree to a cut and done salary reduction to accommodate these furlough days, and this is legally permissible and wont lose you your exempt status. This also strengthens the argument that you didn't simply take a hit for the furlough days, you simply agreed to cut your own salary in expectation of business downturn. Basically there is a quid-pro-quo in having these days off but only if you'd remained working for the company. You'd need to speak to a specialist in wage law to determine if you're entitled to back pay at this point, but you should ask the company about whether they can give you back pay since you're leaving first. It'll also depend on what you agreed to in writing.