http://bugs.winehq.org/show_bug.cgi?id=58661
--- Comment #17 from Stian Low wineryyyyy@gmail.com --- (In reply to Stian Low from comment #16)
Confirming that icu.dll is indeed part of ge-proton packages which allows ILSpy to launch.
Missing icu.dll for vanilla may be solved soon so that ILSpy is also easily reproducible without ge-proton:
https://gitlab.winehq.org/wine/wine-icu