Any compensation deferred pursuant to a plan established under this chapter shall be deemed regular compensation for the purpose of computing contributions or benefits under existing retirement, pension, or social security systems applicable to participating employees; provided that, at the participating employee‘s option, all or part of the deferred compensation may be included or excluded in the computation of federal income taxes withheld on behalf of the participating employee, allowing for both a traditional contribution and a Roth contribution under section 457(b) of the Internal Revenue Code, as amended.