No, holsters with thigh straps are considered to be tie-down rigs and are prohibited per the rules (5.2.7.1, and see definition of ‘tie-down rig’ in App. A3). If you can detach the strap from the holster, you can still use that holster minus the strap. Just make sure that the heel of the butt of the handgun is even with or above the top of the belt (see this post for more info).