I have some old web content that works using Server Side Includes and I needed to get it functional on a new web server. I found this article How do I enable server-side includes (SSIs) on my site? which had everything I needed to know.
First I needed to add the content type and output filter to the Apache configuration:
AddType text/html .shtml AddOutputFilter INCLUDES .shtml
Then I needed to add the IncludesNoExec option:
Options MultiViews Indexes SymLinksIfOwnerMatch IncludesNoExec
Then I needed to specify the DirectoryIndex:
DirectoryIndex index.shtml index.html
And that was it!