@Sciolistes the restriction is not IAN in itself, it is the approach mode. The restriction applies to all approaches that use a curved approach path, no matter if precision or non precision.
For the last 6 years or so we fly every instrument approach exclusively in approach mode, no matter if non precision or precision. It is in so far easier as the same steps, indications and procedures are used completely independent of approach type. Of course pilots have to know the restrictions, but that is nothing new in itself, we had to do know that before as well. LNAV/VNAV was only used as a backup mode and in reality if IAN wasn't available LNAV/VNAV wasn't either so the real backup mode was V/S and basic modes. Never had to use that in the real world for the last 12 years though, only every three years in the simulator.
For those RNP (AR) approaches we use (all of them have a curved approach path) we now have to use a procedure which is uncommon and rarely used, which needs additional steps to make it work or constant FMC inputs (not all planes have SPD intervention) and all in all is just an additional hassle. Curved precision approach paths can not be used at all since the auto flight system is not capable of doing so. Although all our 737 come GLS equipped at no extra cost we can only use half the capability.
Just would like to see an approach mode that can use curved approach paths, APV capability would be nice as well.