[rzzzt]

I have also dug around a bit to find out this one, and the earliest httpd I could get my hands on is 1.3.0 which is hosted on the Apache archive site: https://archive.apache.org/dist/httpd/

"src/modules/standard/mod_include.c" says:

  /*
   * http_include.c: Handles the server-parsed HTML documents
   * 
   * Original by Rob McCool; substantial fixups by David Robinson;
   * incorporated into the Apache module framework by rst.
   * 
   */

Rob McCool is the author of NCSA HTTPd so it seems there is direct lineage wrt. this feature between the two server implementations.