The LibreOffice guys have told me hat ICU has security concerns and is, for all intents and purposes, no longer being developed. They are switching to another engine (hard buzz? Name escapes me).
HarfBuzz and ICU are very different things. HarfBuzz is a small library for text shaping (basically, putting font glyphs together to form words, which can be quite complex for some scripts). ICU on the other hand can do pretty much everything that's vaguely related to internationalization. It's quite possible that LO was only using it for text shaping of course.
Anyone know if this is true?