Surely you can still find some old site from somewhere that uses it, but has someone actually used it even somewhat lately & what use case you had? For most cases where it made sense you would probably just use some static site generator and deploy different version if required. Or do the replacing during the publishing phase.
PHP is totally overkill, and even a site generator feels overkill for this purpose; now I can just throw together some html and upload, no need for running a script.
Also it's a bit of nostalgia (the reason I implemented it a cople of years ago) since ssi was my first contact with web development back in '96 or whenever :)
There is a pure static mode, when you pre-generate all the HTML.
Then there is a hybrid mode, with basic templating of time/clock. SSI can also be used to update pages.
When PHP is available, it can very closely approximate a dynamic site, with pages being pre-generated just in time, but then served to all subsequent requests.