I work for a California municipality where if a holiday falls on a normal day off, then we accrue the day to be used for another date. Holiday accrual does not cap out. Upon termination, does this have to be paid back to the employee like unused vacation time, or is it forfeited?

This sort of thing will be determined by policy in your specific municipality, and is usually published in an employee handbook.
I know of no state law, and certainly no federal law, that would cover this.
If not published, personnel won't lie to you, so just ask.
Sometimes there are restrictions such as how long employment lasted.

There are no laws that require an employer to offer paid time off, period. So any benefit that an employer might offer is subject to their own policies.

If the policy of the employer is to pay out accrued time off, then this should be paid unless holiday accrual is specificalyl excluded or tracked and maintained separately.