The simple answer is that in MOST Pacific airspace (basically everything between the US West Coast all the way out to Japan and down south most of the way to Australia and NZ) no, it is not required. ADS-C and CPDLC used over SATCOM is the essence of the FANS system, and each component is filed as a separate equipment code on an ICAO flight plan. Like others have said it what it really changes is your separation requirements. How well equipped you are on a given day either qualifies or disqualifies you from the various classes of RNP airspace a.k.a. "Performance-Based Navigation". There is a separate section in the ICAO flight plan to file your PBN codes, which will determine the services you will receive. With ADS-C ATC will probably know you are off course before you do! Without it they can't see you so you are back to position reports and estimates, probably over HF radio. Expect to be given non-optimal routings and altitudes, but yes there are operators still flying without the fancy stuff.
Don't let anyone make this stuff out to be more complicated than it really is. CPDLC is basically glorified text messaging, and ADS-C is like a satellite based transponder on steroids and crack! FANS is a wonderful thing; in modern oceanic flying the godforsaken squeal and hiss of HF (aka HAM radio) has been replaced by the blessed silence of SATCOM datalink.