Twine Version: twee3
Story Format: Sugarcube (but any other too, I guess)
Does twee3 use the special passage StoryIncludes? I haven’t found any other instruction on using multiple code files (besides the @include directives).
The Cookbook mentions StoryIncludes as having to do with Twine 1 and this confuses me as to whether it still applies to twee3.
The Twee 3 Specification linked from the cookbook does not mention StoryIncludes or @include or anything similar.
The documentation for the Tweego tool (the only program I know of that compiles Twee 3 files other than Twine itself) suggests that you can just give it multiple files on the command-line, or give it a directory and it will collect all the Twee and related files it can find on its own.
So if you want to break a story in Twee 3 format across multiple files, I’d suggest sticking them all in a directory together rather than having one try to include the others.