conf:

LogLevel trace8
<If "%{CONTENT_TYPE} =~ /.+/">
  Header always set "IFMATCHED" "true"
</If>
Header always set "HEADEREXPRMATCHED" "true" "expr=%{CONTENT_TYPE} =~ /.+/"


$ wget -qS http://localhost/index.html
  HTTP/1.1 200 OK
  Date: Sun, 27 Feb 2022 03:35:49 GMT
  Server: Apache/2.4.53-dev (Unix)
  HEADEREXPRMATCHED: true
  Last-Modified: Mon, 16 Nov 2020 01:43:30 GMT
  ETag: "2d-5b42f81575480"
  Accept-Ranges: bytes
  Content-Length: 45
  Keep-Alive: timeout=5, max=100
  Connection: Keep-Alive
  Content-Type: text/html