Job Benefits Can an Employee Lose Access to a 401k

My question involves labor and employment law for the state of: California

Many employers offer 401k plans to their employees. Some employers require that employees meet specific requirements before being allowed to participate in the 401k.

The IRS limits the requirements that employers can impose on employees with respect to 401k access. Specifically, the most stringent requirement allowed is one year of service + 1000 hours of work.

https://www.investopedia.com/univers...ifiedplan2.asp

Imagine the following situation:

Year 1: Employee works for 1000 hours
Year 2: Employee joins 401k. Employee works for 500 hours.
Year 3: Employee is removed from 401k because he did not work for 1000 hours in year 2.

Is this legal? Can the 1000 hour requirement be imposed for each preceeding year of employment or is an employee who is given access to the 401k entitled to retain the ability to contribute as long as they remain employed?

If the answer is the latter (employee keeps access to 401k), are there any links or documents that address this? any specific terminology that can be searched for?

Job Benefits Can an Employee Lose Access to a 401k