${EXCLUDE_DEPRECATED_BEFORE_AND_AT} is not defined by default.

You want to add an option (or actually cached cmake var) in the toplevel CMakeLists.txt, lke mentioned in the docs: https://api.kde.org/ecm/module/ECMGenerateExportHeader.html
Both to document this flag, as also ensure there the variable is set (did cmake not fail for you?):

set(EXCLUDE_DEPRECATED_BEFORE_AND_AT 0 CACHE STRING "Control the range of deprecated API excluded from the build [default=0].")