I agree with prior comments.
Unlikely they elected ERISA status so universal availability doesn’t apply.
A non-ERISA Church plan isn’t required to have a written plan unless it’s a 403(b)(9) plan - which you stated is what this plan is.
Example 4 from the IRS web site on the remedial amendment period seems to be on point assuming the plan since 2009 - or if later, when established - has had this problem. The example is based on the UA exclusion of 20 hours per week but I don’t think that makes a difference here. You’re subject to EPCRS and if you want to correct by a plan amendment then it’s VCP and there’s no telling if the IRS will allow it. At least with a non-ERISA plan you don’t have to worry about any anti-cutback issues.
Example 4: compliance in form but not in operation
Plan D, adopted effective in 2012, provides that all employees may make elective deferrals. However, the plan sponsor operates the Plan in plan years 2012 - 2016 to exclude employees who work fewer than 20 hours per week (part-time employees) as otherwise permitted under Treas. Regs. Section 1.403(b)-5(b)(4)(ii)(E).
Conclusion: By excluding employees who work fewer than 20 hours per week during 2012 - 2016 without having a plan provision that permits that exclusion, the plan sponsor failed to operate the plan according to its terms. Therefore, Plan D doesn’t comply with Treas. Regs. Section 1.403(b)-3(b)(3)(i). The plan could have been drafted to exclude part-time employees, but wasn’t. Although Plan D, as drafted, complies with the 403(b) regulations in form, it wasn’t operated consistent with its terms and can’t be corrected by amendment during the plan’s RAP. Instead, the sponsor must correct this operational failure under EPCRS.
https://www.irs.gov/retirement-plans/self-correct-defective-403b-plan-provisions-during-the-remedial-amendment-period
