New plan, no contributions in first year

Plan wants to start 1/1 of the next year (1/1/22) for both 401k and PS.

With the plan being adopted prior to 1/1, is there a reason the plan cannot make the PS effective the prior year (1/1/21) even though there is currently no intent to make such a contribution.

What if they have an existing SEP in current year and made the plan effective 1/1/21, would there still be an issue with no contributions for the plan year to the PS?

What is the reason that they want to make it effective 1/1/2021?  Just so they can potentially decide to make a contribution for 2021 at a later date?  If so, then I think it works.  If it is for some other reason, maybe yes, maybe no.

Reason is for potential grandfathering of plans adopted prior to 1/1/22 in proposed bill in Washington.

What advantage do you see to the grandfathering?

No need to add auto enrollment.