The TRX pin is pulled to ground in TX and is open in RX.
The following is true if the reference design is used and no other adjustments are due except for cutting the TRX line.
For RX not connecting the TRX pin will have minimal impact on performance since the source impedance for the LNA will be the same.
For TX the load impedance will change a lot since C181 has a large impact on the impedance. The output power and harmonics will be different.
The TRX pin has to be connected when the RX and TX paths are combined. If RX and TX are split in two independent paths connection to the TRX pin may be omitted.