I mean, if you really want to add a custom function or two to Jekyll you can, but you hardly need to wait around for "support" for this.
You may be asking for a template that has this built in, but you're just as well off to take an existing template you like and modify the text it is generating than to wait around for someone else to do it.
If it seems like the static site generator doesn't "support" this it's because it's just so "what it does" that there isn't any documentation for how you'd specifically put SSI tags into a site, because that'd just like expecting specific documentation for how you put <h2> tags into a website. It's just what it does already.