Is the supplemental executive retirement plans SERP taxed before the benefit is received?

This can be a very complex field, some basics:

All amounts deferred under a nonqualified deferred compensation (NQDC) plan for all tax years are currently includible in gross income to the extent not subject to a substantial risk of forfeiture and not previously included in gross income, unless the plan:

... meets specified distribution, acceleration of benefit, and election requirements; and

... is operated in accordance with these requirements. ( Code Sec. 409A(a)(1)(A)(i) )

If a NQDC plan doesn't comply with the Code Sec. 409A rules, all amounts deferred under the plan for the tax year and all prior tax years, by any participant to whom the failure relates, are included in income for that year to the extent not subject to a substantial risk of forfeiture and not previously included in income. This amount is also subject to: (1) interest (at the underpayment rate plus one percentage point) on the tax underpayments that would have occurred had the amount been included in income for the tax year when first deferred, or if later, when not subject to a substantial risk of forfeiture; and (2) a penalty of 20% of the compensation required to be included in income. ( Code Sec. 409A(a)(1)(B)