I cannot speak for Ruby core team but I personally do not like ICU.

It's not that I am against Unicode but Unicode is only part of what
Ruby aims to support yet ICU is huge and written in C++ which would
drastically enlarge Ruby core size and reduce portability.

Better support for character classes outside of the ASCII range is
certainly desirable but it requires more design and planning than
"Hey, I saw ICU, it's cool, let's use it".

First the questions

 - What exactly we want supported for what purposes?

 - What the cost would be?

 - Do we really want to pay that cost?

have to be answered.

There are clearly multiple options and I haven't gathered enough data
to even make an opinion what would be good option for Ruby.

Thanks

Michal