In https://httpd.apache.org/docs/2.4/expr.html there is the expression filemod listed. But this has only been added to trunk https://github.com/apache/httpd/blob/700d9b5b5bc710021e978c95ef271e49bc79cc89/server/util_expr_eval.c and is not backported to the 2.4 branch.
Fixed in r1898351.