the old Area frequencies become just ATC low level frequencies
Not "old" - they are current
The FIA's are declared volumes of airspace for exactly that purpose i.e. Flight Information Areas, on which ATC's provide ATS.
All things considered it might have been easier and safer to just remove the poorly thought out requirement to use the Area Frequency at 'unmarked' airfields, making the MULTICOM the default low level frequency outside of BA's and CTAFs with a different freq - with no dimensions, just recommended practices
And simpler
