Comment 1 for bug 1865340

https://httpd.apache.org/docs/2.4/mod/mod_proxy_ajp.html seems to indicate "secret" will be available in 2.4.42:

?secret 0x0C String Supported since 2.4.42

From https://bugzilla.redhat.com/show_bug.cgi?id=1397241, looks like redhat has had "secret" support for quite a while. That bug report links to this changeset:

https://svn.apache.org/viewvc?view=revision&revision=1738878

Looks like this is the 2.4.42 commit:
https://github.com/apache/httpd/commit/d8b6d798c177dfdb90cef1a29395afcc043f3c86

With a follow-up doc update:
https://github.com/apache/httpd/commit/4de7604dd086c7bebdcab4ae9dbbec24b59edabc

I grabbed the above from the 2.4.x branch