This is not true. Plasticizers are generally not used in elastomeric seals (nor tires as in your previous reply). Elastomers (of which natural rubber is a member) embrittle due to oxidation and thermal degradation. In the case of polyurethane elastomers, water hydrolysis can cause degradation.

Leaving a dishwasher unused will actually prolong the life of the seals since there is less exposure to water, oils, and oxygen at high temperatures.