We were using Nordic HAL for FEM modules to drive our PA/LNA using nRF52840 on zephyr (no NCS). It was only a basic configuration, where the PA and LNA pins were selected using simple_gpio library present on hal_nordic module with calls to nrf_fem_interface_configuration_get() and nrf_fem_interface_configuration_set() control functions.
With introduction of opensource MPSL on v2.5.0, I couldn't find the control function on the MPSL or anywhere on hal_nordic module, and the old functions were removed. It seems that nrfxlib used by nRF Connect SDK provides a proprietary (compiled) MPSL library. This could explain the "opensource" name on the SL folder on hal_nordic. Links: 1, 2.
Does Nordic pretends to bring back support to FEM control function on zephyr through its MPSL opensource or any other module/library? The hal for FEM on nrf52840 involves a lot of details, e.g. PPI, timers, interrupts, etc and... it was already there before zephyr v2.5.0, working fine.