In Missouri, your ESA housing rights rest first on state law. The Missouri Human Rights Act (§ 213.040 RSMo) makes it an unlawful housing practice to discriminate because of disability and requires reasonable accommodations in rules, policies, and practices when they are necessary for equal use of a dwelling. This state protection is enforced by the Missouri Commission on Human Rights and remains fully active. The federal Fair Housing Act (42 U.S.C. § 3604) provides the same obligation, and that statute has not changed.
Landlords in O'Fallon who refuse a valid ESA letter or continue charging pet fees may be in violation of the Missouri Human Rights Act and federal law. Complaints can be filed with the Missouri Commission on Human Rights.