Don't know about IPC but UPC lists approved fittings that may be concealed "fittings such as tees elbows and right left nipples and couplings" other codes list fittings that are cast as being prohibited.
My experience and observation has been bell reducers are ok to use anywhere, bushings ok in un concealed spaces. gas company routinely uses them (bushings) at meter locations.
Been a bit of gray area to me don't know if a street 90 is "cast" or not but had some discussion in past on using for gas I used them many times but uncertain of legality, we used them (street 90s) occasionally on log lighter connections. I never use bushings buried in wall just because of controversy