It was introduced in c18051e781. It didn't work as expected.
c18051e781
@ -411,8 +411,6 @@ termux_step_start_build() {
echo "$TERMUX_PKG_NAME@$TERMUX_PKG_FULLVERSION built - skipping (rm /data/data/.built-packages/$TERMUX_PKG_NAME to force rebuild)"
exit 0
fi
else
export TERMUX_PKG_REPLACES=${TERMUX_PKG_NAME}
# Cleanup old state: