You’re not the first one to ask that feature. Implementing it properly is trickier than it seems, since we would need to write a language server. (There’s this one, but it’s a bit buggy and not maintained anymore, as far as I know.)
It would be possible to implement the outline by using regexes (say finding all the lines starting with [ to find routines, and so on) but it would be approximate (it won’t work with multiple declarations on the same line). Also, object declarations start with an arbitrary word (the name of a class), which might not be easy to find reliably.
Anyway, I wrote a more detailed explanation here (my answer is in English, don’t mind the French):
I’m still tempted to write an Inform 6 language server because it would be a fun project (I guess), but since I’m not really an Inform 6 user, it’s not really high in my project list.
In the meantime, I might go the regex way, if there’s demand. (But again, not my priority. I still have to update the Inform 7 extension for 10.1, and I don’t have much time right now.)