"Proposed" is the final state of many stable RFCs; see a list at [1], and compare it to the (much shorter) list of finished standards above it. The name is kind of misleading these days. Some discussion at [2]. Just to give an example, the base64 RFC 4648 is also a "proposed standard".

[1] https://www.rfc-editor.org/standards#PS

[2] https://datatracker.ietf.org/doc/html/rfc7127#section-2