"I'm not perfectly clear on how you would test this design, and would appreciate any insight. A 401(k) plan allows immediate eligibility for deferrals on date of hire; the only condition is age 21.

The plan also provides a safe harbor nonelective (also age 21). Eligibility for this portion is the January 1 or July 1 after working 1000 hours (not January 1 or July 1 following a full 12-month period in which the employee completed 1000 hours). So, if a full-time employee was hired on July 1, 2021, they may work 1000 hours before the end of 2021 and enter the safe harbor portion on January 1, 2022.

This does not seem (to me at least) to impose the maximum permissible minimum age and service conditions in 410(a), so it's not clear that dividing line for ADP/safe harbor would necessarily correspond to the participants actually getting those contributions when using the otherwise excludable employee rule (i.e., some participants who have not satisfied the maximum permissible age and service requirements would be getting safe harbor nonelectives).

If that's the case, how would you test? Would it be everyone with less than the maximum permissible conditions subject to ADP (even if some are getting safe harbor nonelectives) and everyone with more than the maximum permissible conditions exempt from ADP testing due to the safe harbor? FWIW, this is what ERISApedia and Who's the Employer seem to suggest."