A completely non-authoritative answer...
RNAV 10 is an (older) oceanic spec. So, picking a "far away FIR", I googled "Are airways in Mauritius RNAV or RNP?"
Mauritius has implemented a mix of RNAV and RNP within its airspace.
Upper airspace utilizes RNP 10, which aligns with RNAV-10. Additionally, Mauritius is moving towards implementing RNP 4 in its oceanic airspace
Feel free to replace any other FIR/UIR to replace Mauritius in your search...
I imagine that most airways are moving towards an RNP standard. Several years ago, I found it very difficult (from the relevant chart legend) to determine whether an airway we were flying on was RNP or RNAV. Nothing seems to have changed.
Also, code7700.com gives the following:
We say almost always because RNP-10 is an exception. Many areas of the world adopted "RNP-10" as synonymous with "RNAV-10," violating the method ICAO set up in ICAO Doc 9613 Performance Based Navigation. The ICAO elected to retain "RNP-10" when in fact it meant "RNAV-10" because changing all the charts and manuals would have been too expensive. The key takeaway from all this is to realize that "RNP-10" does not require performance monitoring and alerting, as do the other RNP designations.?
And an official but dated(?) link
here.
Good luck.