That's because the structure of the docs changed. You need to hit "up", which will take you to https://www.postgresql.org/docs/current/client-authentication.html, which now has the list of links. Note how the different methods used to be 20.3.x, and are now directly listed as 20.y.
I'm unsure if that was intentional in the upstream docs, but that's what makes the website behave like it does.
Fair enough but
20.3. Authentication Methods
The following sections describe the authentication methods in more detail.
certainly is misleading.
This was changed by Peter in commit 56811e57323faa453947eb82f007e323a952e1a1 along with the restructuring. It used to say "the following subsections". So techically I think that change is correct, but that doesn't necessarily make it helpful.
But based on how it actually renders, since that section doesn't contain any actual useful info, we should perhaps just remove section 20.3 completely. Peter, thoughts?