More specifically: 1. with LDFLAGS=-Wl,--as-needed no binary links to any of the icu libraries, 2. grepping the sources shows no reference to ICU headers, 3. nor a single reference to HAVE_ICU macro. I'd say ICU pretty much exists in the configure check only and nowhere else. If I'm correct, I suggest dropping USE=icu and disabling ICU support unconditionally (to avoid unnecessarily appending it to LIBS).
ICU is still unused in openldap-2.4.40.
https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=2baad4921bfcff1cc24af346d9257196c705d700
https://bugs.openldap.org/show_bug.cgi?id=9144 https://git.openldap.org/openldap/openldap/-/commit/f8427a9b58436204f62d8c87fdccecc259a2b579