Sony a7 III, a7R III, a9, a6400 will receive Firmware Updates to provide FULL compatibility for Sony RMT-P1BT Wireless Remote Commander. The firmware updates add full functions, but in the mean time, all four cameras will have preliminary support.
Sony a7 III & a7R III Firmware Updates scheduled for April 2019
Sony a9 & a6400 Firmware Updates scheduled for Summer 2019
Sony RMT-P1BT Wireless Remote Commander available for Pre-Order 10 am ET Feb 28 from B&H Photo