Severance

Can an employee make a deferral into the plan with money received as severance before the plan termination date or can it be made even after the termination date? Also, does the employer has to put in a match (Safe Harbor) on the severance pay? 

Retirement plans, of several kinds, have a range of provisions about whether (and the extent to which) severance pay counts in compensation from which a participant might make an elective deferral, or in compensation on which an allocation of a matching or nonelective contribution is determined.

As BenefitsLink neighbors say, RTFD—Read The Fabulous Document.

Also, if your query relates to an end or discontinuance of a plan, consider how a plan amendment might affect provisions about counting or not counting severance pay.

Post-severance compensation has specific defined meaning(s) under the IRC and should be covered in the plan document as to what is and is not included.

True severance pay - money paid upon/after termination of employment on account of such termination that would not have been earned/paid under continued employment is not plan compensation in the eyes of the IRS and cannot be considered for retirement benefits or contributions.