RX & TX are still connected to UART0, and still used with the USB when in CDC (download mode). Otherwise, you can use them as UART0 when connecting a module that uses serial, or use them as general IO, it’s up to you.
On the TinyPICO Nano, RX/TX are tied to the SerialUART chip and cannot be used for other purposes. They are exposed so you can read the serial output from UART0.