Perhaps an issue on the collation page?

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/15/collation.html
Description:

This page states:

Observe how in the traditional ICU locale naming system, the root locale
is selected by an empty string.

CREATE COLLATION latinlast (provider = icu, locale =
'en-u-kr-grek-latn');
CREATE COLLATION latinlast (provider = icu, locale =
'en(at)colReorder=grek-latn');

Neither of the locales are empty strings.