The reference to RFC 3280 in Part 6 was updated to RFC 5280 in Issue 0006383. This is relevant because of clarifications made in the newer release regarding the use of self-signed certificates as end-entity certificates.

From https://tools.ietf.org/html/draft-ietf-pkix-rfc5280-clarifications-11:
| Consistent with Section 3.4.61 of X.509 (11/2008) [X.509] we note
| that use of self-issued certificates and self-signed certificates
| issued by other than CAs are outside the scope of this specification.
| Thus, for example, a web server or client might generate a self-
| signed certificate to identify itself. These certificates, and how a
| relying party uses them to authenticate asserted identities, are
| both outside the scope of RFC 5280.

The RFC never covered the way self-signed certificates are often used in OPC UA applications and made this clear in the updated RFC.

Since the security working group recently had discussions about the contents of self-signed end-entitiy certificates (the CA flag in particular) and the RFC explicitly does not cover the use case, the OPC UA specification should cover these certificates in more detail.