I remember hearing at some point that workers in the U.S. who are working on a H1B Visa and who will leave the use when the H1B expires can reclaim social security and medicare taxes back. Does anyone know if this is true or false?

False.

Workers on H-1B visas are subject to these taxes and they will NOT be refunded.

You MAY get credit for these taxes in your home country's social security system if they have such an agreement with the United States.