There are a lot of things I have seen in Unicode that seem like they should not exist in the first place. MATHEMATICAL [BOLD|SANS-SERIF|DOUBLE-STRUCK|MONOSPACE] DIGIT for example... I guess those things potentially carry significant meaning in some mathematics texts though.
I guess the ICU stuff probably gives you an strtol equivalent that can handle that sort of stuff.
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.
I guess the ICU stuff probably gives you an strtol equivalent that can handle that sort of stuff.