It's definitely proprietary at the physical layer. You cannot make your own LoRa RF chips. It's patented, closed-source, and owned by Semtech. There is no public specification.
However, the higher layers above physical are open and the proprietary radio chips are well-supported and widely available to hobbyists. There's not much else in the low-power, low-cost, and no-license required to transmit niche, and it works really well at what it aims to do.