While building the CLR on macOS Big Sur beta (10.16, or 11) the build fails with the following error.

In file included from /Users/kjones/Projects/runtime/src/libraries/Native/Unix/System.Globalization.Native/pal_casing.c:8:
  /Users/kjones/Projects/runtime/src/libraries/Native/Unix/System.Globalization.Native/pal_icushim_internal.h:184:1: error: 'ucol_setVariableTop' is deprecated [-Werror,-Wdeprecated-declarations]
  FOR_ALL_ICU_FUNCTIONS
  ^
  /Users/kjones/Projects/runtime/src/libraries/Native/Unix/System.Globalization.Native/pal_icushim_internal.h:179:5: note: expanded from macro 'FOR_ALL_ICU_FUNCTIONS'
      FOR_ALL_SET_VARIABLE_ICU_FUNCTIONS \
      ^
  /Users/kjones/Projects/runtime/src/libraries/Native/Unix/System.Globalization.Native/pal_icushim_internal.h:156:24: note: expanded from macro 'FOR_ALL_SET_VARIABLE_ICU_FUNCTIONS'
      PER_FUNCTION_BLOCK(ucol_setVariableTop, libicui18n)
                         ^
  /usr/local/opt/icu4c/include/unicode/ucol.h:1270:1: note: 'ucol_setVariableTop' has been explicitly marked deprecated here
  U_DEPRECATED uint32_t U_EXPORT2 
  ^
  /usr/local/opt/icu4c/include/unicode/umachine.h:115:29: note: expanded from macro 'U_DEPRECATED'
  #define U_DEPRECATED U_CAPI U_ATTRIBUTE_DEPRECATED
                              ^
  /usr/local/opt/icu4c/include/unicode/umachine.h:95:52: note: expanded from macro 'U_ATTRIBUTE_DEPRECATED'
  #    define U_ATTRIBUTE_DEPRECATED __attribute__ ((deprecated))
                                                     ^
  1 error generated.

I'm able to work around this by specifying -Wno-deprecated-declarations to the compiler for now.