Server Side Include (SSI) is a specific concept and related feature of web servers. See https://en.wikipedia.org/wiki/Server_Side_Includes
When was this feature first introduced?
The Wikipedia article references a Wayback Machine entry from September 1995, but I know I used this feature sometime between 1992 and February 1995, and that it was likely before Common Gateway Interface came along in 1993. The feature was likely introduced in NCSA HTTPd.