Mo. Rev. Stat. § 57.988: Exemptions from taxes of state and political subdivisions, legal process and claims — rights unassignable, vested.

1. Any annuity, benefits, funds, property, or rights created by, or accruing to, any person under the provisions of sections 57.949 to 57.997, are hereby made and declared exempt from any tax of the state of Missouri or any political subdivision or taxing body thereof, and shall not be subject to execution, garnishment, attachment, writ of sequestration, or any other process or claim whatsoever, and shall be unassignable.

2. No alteration, amendment, or repeal of the provisions of sections 57.949 to 57.997 shall affect the then existing rights of members and beneficiaries, but shall be effective only as to rights which would otherwise accrue hereunder as a result of services rendered by an employee after such alteration, amendment, or repeal.