A quick scan of the source code shows SRXL, but not SRXL2.
In case this helps, you mention the transmitter in regard to this. I see that SRXL2 is indeed a receiver output protocol, but if you are mainly interested in using the transmitter, perhaps you can use a different receiver that outputs something that LP can use.
If I might suggest, it looks like Ardupilot supports SRXL2. Ardupilot seems more actively developed than LibrePilot
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Ardupilot says: "All Spektrum receivers released since August 2019 only support the SRXL2 protocol."
I would add that any receiver that you can plug a set of standard (analog or digital) servos into supports PWM (the standard, one servo per wire protocol), which LP does support as long as your receiver has a set of servo (PWM) outputs, and your flight controller has a set of PWM inputs.