That post says that they currently work, but there is a new os-independent collation method coming.
Edit re your edit: the already working collations are in PG 9.x - the OS independent collations are coming in 10.x. (Hence the post title, "More robust collations with ICU support in PostgreSQL 10")
But do these "already working" collations include accent-insensitive variants? I didn't think so, and I don't think they are coming in PostgreSQL 10 either. Do you have other information on this?
They do. The problem is that equality is still strict binary equality for text, for obscure technical reasons. So, while you can get extremely flexible customizations to collation with icu [1], you can't have case insensitive equality, or equality that ignores accents. This is recognized as a problem, and is probably going to be fixed in v11.
My misunderstanding seemed to be that "accent-insensitive" turns out to be a precise technical term from MySQL that says "just sort UTF-8 using the American locale and collation, but pretend accented characters are equal to their non-accented versions" which is not a standard collation order in any locale.
