https://www.postgresql.org/about/news/1972/
=> PostgreSQL 12 Beta Release Notes
https://www.postgresql.org/docs/devel/release-12.html
=> 404
They were removed at:
commit 615cebc94b5ef8fbe353e3c8b838b1e97bcdfd49
Author: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Date: Mon Jul 1 12:50:55 2019 -0400
(Arguably it would've been better if the v12 notes were maintained in "master"
and any commits cherry-picked to REL12 until after release, since the devel's
web docs are updated frequently).
Thanks,
Justin