Archive.org tells me I was using SSI in Jan 1997. I didn’t really understand what I was doing, but including the footer and a visitor counter via an exec one which I presumably copied from somewhere else. At the time I was still on windows and had no real concept of a program being executed as a cgi or ssi, it was all “copy this from Matt’s script archive to your cgi-bin directory”

My shared hosting from claranet supported ssi via a .htaccess configuration.

Technically php was around at that point, but I don’t think it became popular until php3 - certainly my hosting provider didn’t support it until then.