Which Navigation Specifications support an RF leg?
First is understanding the difference between RNAV and RNP.  This related question: What is the difference between RNAV and RNP? provides a basic answer to that part of your question.  

To your specific question, RNAV procedures predate the acceptance of RNP standards. Since RNP standards have been established as the basis of navigation, you should expect that all procedures published in the future would be RNP procedures.  

As a reference, I can point to RTCA Document DO-236C, Minimum Aviation System Performance Standards: Required Navigation Performance for Area Navigation (MASPS). This document describes how RNP standards will be applied in the airspace. That covers airspace design, procedures and equipment used to fly within RNP airspace. Performance Based Navigation (PBN) adds aircraft specific performance requirements to the RNP concept. 

Definitions from DO-236C which references ICAO Doc 9613:  


  RNAV system (as defined by ICAO Doc 9613): A navigation system which
  permits aircraft operation on any desired flight path within the
  coverage of station-referenced NAVAIDs or within the limits of the
  capability of self-contained aids, or a combination of these. An RNAV
  system may be included as part of a flight management system (FMS).
  
  RNP system (as defined by ICAO Doc 9613): An area navigation system
  which supports onboard performance monitoring and alerting. An area
  navigation system capable of achieving the performance requirement of
  an RNP specification is referred to as an RNP system.  
  
  PBN (as defined by ICAO Doc 9613): Area navigation based on
  performance requirements for aircraft operating along an ATS route, on
  an instrument approach procedure or in a designated airspace.  


Also from DO-236C, section 3.2 Path Definition Requirements, we find:  


  3.2.1.1 Navigation Database Flight Path Legs for Terminal Area Procedures
  
  Permissible leg types for the definitions of the RNP flight paths for
  all navigation database procedures (e.g. SIDs, STARS, Approaches
  including missed approaches), with the exception of airways, are
  limited to the following:
  
  Permissible Leg Types          ARINC 424 Leg Type
  - Track to Fix leg                      TF
  - Radius to Fix leg                     RF
  - Initial Fix                           IF
  - Hold legs                        HM, HA and HF
  - Fix to Altitude leg                   FA
  - Direct to Fix leg                     DF
  - Course to Fix leg                     CF
  
  Note 1: RNP procedures that require the repeatability and predictability of
  a specified ground track should be constructed using IF, TF, and RF legs only.
  
  Note 2: If the FA, DF and CF types are used, they may introduce less
  repeatability and predictability.


As you can see from the above, RNP procedures are built using RNAV leg types. RF legs are an RNAV leg type unlike the similar AF leg which is a DME arc that can be flown with a DME.   

Realistically, you will only find RF legs within SIDs, STARs, and IAPs as that's where they are needed. They are useful when you need a large change of direction which is common in the terminal environment but not so much while enroute.