Even though icu-le-hb only has dev-libs/icu as a DEPEND, and hence no sub-slot dependency, I found that games-simulation/openttd would fail to build following the ICU 59 update until I had rebuilt icu-le-hb. It doesn't have a direct shared library dependency on ICU, only a transitive one via HarfBuzz, but there must be ABI-dependent stuff buried in there somewhere. Unfortunately I have lost the errors I received but they were undefined references.
I was hit by the same issue. I still wonder if we can put dev-libs/icu:= into RDEPEND for this reason.
commit f8949c177693d93f0a88eab74e2e170d32bbc634 (HEAD -> master, origin/master, origin/HEAD) Author: Lars Wendler <polynomial-c@gentoo.org> Date: Fri Jul 21 11:14:32 2017 dev-libs/icu-le-hb: Move dev-libs/icu to REDPEND (bug #621786). Package-Manager: Portage-2.3.6, Repoman-2.3.3