it's easy to build automatically during the build the exhaustive list of removed files with a command like
git diff --summary <old_version> <new_version> |grep 'delete mode '|grep phpBB/ |sed 's/delete mode [0-9]* phpBB\///g'
diff -qr new_version/phpBB3/ old_versions/<old_version>/ |grep 'Only in old_versions/<old_version>/'|sed 's/Only in old_versions\/<old_version>\///g'|sed 's/: /\//g'
a list which could be added in the update package and then parsed and displayed at the end of the update process.
what do you think?