All ports are using ComplexTextController, and should support expansion around ideographic characters. So it seems like we could remove FontCascade::canExpandAroundIdeographsInComplexText, but trying that in attachment 446238 [details] resulted in a failure in a ruby test on Windows.
The failure: https://ews-build.webkit.org/#/builders/10/builds/116242
<rdar://problem/86497764>