I have no idea how I missed that, but: if it wasn't for Alvaro's blog post I wouldn't know that: on 7th of April 2020, Alvaro Herrera committed patch:
Support FETCH FIRST WITH TIES
WITH TIES is an option to the FETCH FIRST N ROWS clause (the SQL
standard's spelling of LIMIT), where you additionally get rows that
compare equal to the last of those N rows by the columns in the
mandatory ORDER BY clause.
There was a proposal by Andrew Gierth to implement this functionality in
a more powerful way that would yield more features, but the other patch
had not been finished at this time, so we decided to use this one for
now in the spirit of incremental development.
Author: Surafel Temesgen <firstname.lastname@example.org>
Reviewed-by: Álvaro Herrera <email@example.com>
Reviewed-by: Tomas Vondra <firstname.lastname@example.org>
Continue reading Waiting for PostgreSQL 13 – Support FETCH FIRST WITH TIES