The NAT RFCs came after the routing RFC and refer to NAT as a router function not as an orthogonal function, boxes that do NAT are referred to as routers in the RFC. This is reflected in the real world where NAT is implemented as part of the routing chain not as a separate module. Remember NAT isn't a box creating 2 sockets and ferrying data between them it is just the translation of fields on top of normal routing functionality.
> boxes that do NAT are referred to as routers in the RFC
This is not the case, certainly if you are referring to the router requirements. Last I looked, the rest of the IETF was also very cognizant of the distinction, as there has been wide anti-NAT sentiment in the IETF, trying to get people to move over to IPv6 etc.
Again router requirements came out before NAT, you're not going to find anything in that RFC about something else they hadn't written yet. You have to look at the RFCs for NAT to see they are referred to as routers e.g. starting with https://tools.ietf.org/html/rfc1631:
"2. Overview of NAT
The design presented in this memo is called NAT, for Network Address Translator. NAT is a router function that can be configured as shown in figure 1. Only the stub border router requires modifications."
The IETF collectively aren't big fans of NAT as a good solution but that hasn't stopped multiple standards track NAT RFCs per year. v6 only increased this with all of the transitional mode NAT types (46, 64, 464, 646).
That's an informational rfc that has no weight in this manner, router requirements is a standards track document.
"4.2.2 Informational
An "Informational" specification is published for the general information of the Internet community, and does not represent an Internet community consensus or recommendation. The Informational designation is intended to provide for the timely publication of a very broad range of responsible informational documents from many sources, subject only to editorial considerations and to verification that there has been adequate coordination with the standards process (see section 4.2.3)."
