According to Erik Huizer, informative references to non-standards-track
RFCs are permitted, while normative references are not, in standards-track
This is of course a judgment call, but my guess would be that a sentence
"The ISO 8859 character sets are used as 8-bit character sets without shift
functions, as described in RFC 1345"
would be acceptable, while
"The ISO 8859 character sets are defined by RFC 1345"
would not be.
But this is a judgment call, and would probably have to be decided by the
IESG or IAB if someone really wanted to challenge a specific reference.