It hasn’t been proprietary for a little while, its an official standard now. They had to open it in order to receive government funding for charger construction.

The North American Charging Standard (NACS), being standardized as SAE J3400, is an electric vehicle (EV) charging connector system developed by Tesla, Inc. It has been used by all North American market Tesla vehicles since 2021 and was opened for use by other manufacturers in November 2022. It is backwards compatible with the proprietary Tesla connectors made before 2021. (link)