> Instagram stories (and IG in general) has been functional on the mobile web for more than 3 years

Maybe that's geo-locked? When I visit instagram.com and log in I can only consume content, I cannot upload anything, normal posts nor stories.

Edit: I realize now you said "mobile web" while I was thinking "web" in general and visit from my desktop. Seems weird it works on 1/2 of the web versions they have...

Kind of. There are still settings that are available only in the app, which I ran into recently - can't remember exactly what but eventually I gave up and installed the app.