Short answer: Not as a blanket rule. The EEOC says requiring no medical restrictions violates the ADA if the employee can do the job with or without reasonable accommodation, unless the accommodation is an undue hardship.
A ‘100% healed’ or ‘full duty only’ return policy is risky under the ADA. If the employee can perform the essential functions with or without reasonable accommodation, refusing to let them return because they still have restrictions violates the ADA, unless the needed accommodation would be an undue hardship. Claiming a safety risk requires showing a ‘direct threat,’ a significant risk of substantial harm.
The EEOC’s example: an employee returning from two months of leave asks for an ergonomic chair, adjusted lighting and eight days of part-time work. The employer may not keep her on leave until she can work full time with no restrictions; it may deny a specific accommodation only if it is an undue hardship.
If reassignment is the only option, the employer should place the employee in a vacant position they are qualified for. For FMLA leave, a fitness-for-duty certification is allowed only under the FMLA rules.