In a defined contribution plan:

If forfeitures are used to reduce employer contributions, and the employer makes a contribution that, together with the forfeitures, would exceed the 404 deductible limit, has the employer made a nondeductible contribution? Or can the forfeitures be placed in a suspense account and used to reduce a future year contribution?

Is the answer different for money purchase and target plans as opposed to profit sharing, 401(k), and ESOP, etc.?

Forfeiture doesn't count towards the 404(a) deductibility limit. As long as the deductible contributions (i.e. contributions to the plan excluding forfeiture, post-tax, etc.) are less than 15% for a PS plan or 25% for a pension plan, the 404(a) limit has not been exceeded and there are no nondeductible contributions.