The set of individuals considered affected participants upon a plan termination/partial termination seems rather more extensive for defined benefit plans than for defined contribution plans. To provide some perhaps helpful context:
https://www.irs.gov/retirement-plans/improper-forfeiture-by-defined-benefit-plans
Most defined benefit (DB) plans cannot forfeit a participant's benefit after 5 consecutive 1-year breaks-in-service. However, some DB plans include language providing for a forfeiture of a participant's non-vested accrued benefit either when a participant terminates employment or after the participant incurs 5 consecutive 1-year breaks-in-service. If the plan language forfeits the non-vested benefit upon termination, the plan document must also provide that the plan will restore the forfeited amount if a forfeited participant returns to service prior to incurring 5
consecutive 1-year breaks-in-service. In other words, for IRC Section 411(a)(6)(C), because a nonvested or partially vested participant who has a 5 consecutive 1-year breaks-in-service cannot increase his or her vested percentage in the pre-break-accrued benefit, the plan can forfeit the non-vested accrued benefit that accrued prior to the 5 consecutive 1-year breaks-in-service. However, IRC Section 411(a)(6)(C) only applies to DC or fully insured DB plans.
https://www.irs.gov/pub/irs-tege/2014cpe_plan_terminations.pdf
A defined benefit plan cannot use the 5-year break in service rule to forfeit nonvested benefits unless it is a fully-insured plan. See IRC Section 411(a)(6)(C). Therefore, all employees and former employees with accrued benefits under the plan are "affected employees" for plan termination purposes, without regard to whether they have incurred a 5-year break in
service before the plan's termination date.
https://www.irs.gov/pub/irs-tege/2014cpe_plan_terminations.pdf#page=57
(C) 5 consecutive 1-year breaks in service under defined contribution plan For purposes of paragraph (4), in the case of any participant in a defined contribution plan, or an insured defined benefit plan which satisfies the requirements of subsection (b)(1)(F), who has 5 consecutive 1-year breaks in service, years of service after such 5-year period shall not be required to be taken into account for purposes of determining the nonforfeitable percentage of his accrued benefit derived from employer contributions which accrued before such 5-year period. For DB plans which contain cash-out forfeiture provisions, any former employee whose vested benefit is cashed-out will forfeit the nonvested benefit at that time. A subsequent plan termination would then not result in
any additional vesting for the former employee.
http://uscode.house.gov
For helpful context perhaps, defined benefit plans feature the following vesting: (ii) 5-year vesting A plan satisfies the requirements of this clause if an employee who has completed at least 5 years of service has a nonforfeitable right to 100 percent of the employee's accrued benefit derived from employer contributions. (iii) 3 to 7 year vesting A plan satisfies the requirements of this clause if an employee has a nonforfeitable right to a percentage of the employee's accrued benefit derived from employer contributions determined under the following table:
Years of service: 3 -- 20% 4 -- 40% 5 -- 60% 6 -- 80% 7 or more -- 100%
So, perhaps the set of individuals considered comprising part of the set of affected participants would seem to exclude only those logged as having/having had accounts who had
severed employment prior to the plan termination/partial termination and had not attained 5 years of service (per Schedule I) or 2 years of service (per Schedule II).