A health maintenance contract may contain enrollee cost-sharing provisions as specified in this section. Co-payment and deductible provisions in a group contract must not discriminate on the basis of age, sex, race, disability, economic status, or length of enrollment in the health plan. During an open enrollment period in which all offered health plans fully participate without any underwriting restrictions, co-payment and deductible provisions must not discriminate on the basis of preexisting health status.