ADA compliance means meeting the requirements of the Americans with Disabilities Act in US workplaces: accessible routes, doors, restrooms and workstations, plus reasonable accommodations for employees with disabilities.
Flexible offices add a booking dimension: an accessible desk that anyone can book is not an accommodation. Reserving specific accessible workstations, rooms and parking for the people who need them, by default rather than by request, is how shared-desk models stay compliant in practice.