Solved
Why does B-L072Z-LRWAN1 optionally allow 32MHz clock to PHY_OSC_IN?
According to the user manual (st (dot) com/resource/en/user_manual/um2115-discovery-kit-for-lorawan-sigfox-and-lpwan-protocols-with-stm32l0-stmicroelectronics.pdf), you can optionally close SB13 to get a 32MHz clock signal to the MCU instead of attaching your own external clock. However, when selecting the clock configuration for the STM32L072CZYx the HSE crystal freq is only allowed up to 25MHz and I see similar information in the datasheet for STM32L072xZ (st (dot) com/resource/en/datasheet/stm32l072cz.pdf).
Maybe I am just misunderstanding but it sure seems like the intent is for the TXCO to be routed to the PHY_OSC_IN pin on the MCU in some applications, but the MCU is only rated for 25MHz