I plan on using MPU to mark all memory (SRAM) as XN. I would like to know if this could cause any issue with SoftDevice (iow, may the softdevice execute any code from SRAM?).
Using SDK 15 and nRF52832.
The S132 v6 does not run any code from RAM, so for the time being there should be no ill effects from preventing code execution over the entire RAM region.
Please be aware that this could change in future SoftDevice releases, so if you want to update to a different SoftDevice in the future you have to check this again.