To be clear, this is not an RFC yet. It's not even adopted by a working group, although I hope it will be.
Mods: can RFC be removed from the title? [Edit: thanks for updating the title!]
Mods: can RFC be removed from the title? [Edit: thanks for updating the title!]
Very roughly, a Standards Track document in the IETF goes through four stages:
1. Someone submits an I-D as an individual. Literally anyone can do this and such a document carries no weight.
2. A working group (WG) "adopts" the I-D and proceeds to work on the document as a group. The document still carries no weight at this stage.
3. The I-D is published as an RFC. When this happens, the document finally has real weight. The document can no longer be changed, although it can be updated or obsoleted by another RFC.
4. When the RFC matures, it can be promoted to an Internet Standard.
Let localhost be localhost is still in the first stage.
For example many of you will have used Let's Encrypt, the whole of Let's Encrypt is built upon a standard issuance and management protocol, ACME. But ACME is still only an Internet Draft, albeit it's likely to go to RFC before the end of this year. So it didn't matter that in your view it "has no weight", it had millions of people using it.
If this I-D takes off and is widely accepted by, say, Microsoft and Apple, even if it never becomes an RFC it has a real effect. On the other hand, if it becomes a standards track RFC but Apple decides to ignore it and never promise that "localhost is localhost" in their operating systems then it's worthless despite the "Standard" tag.