For future reference, this actually fixes the build with ICU 61 and up. See the ICU 61 changelog:

C++ call sites need to qualify ICU types explicitly, for example icu::UnicodeString, or do using icu::UnicodeString; where appropriate. If your code relies on the "using namespace icu;" that used to be in unicode/uversion.h, then you need to update your code. (#13460)

Also note that this PR did not fix the problem completely, see #6690 and the fix in 19076c2.

Only #6693 was strictly necessary for 61(.1), as far as I'm aware

I believe it was more a matter of compilation options: All the things fixed by 19076c2 are inside WITH_ICU_SORT, whereas the fixes in #6693 are guarded by WITH_ICU_LAYOUT.

Doesn't matter much, though :-)