I imagine that, like me, you expect it to open a temporary, drop-down or otherwise modal menu like basically every other app and website that ever adopted this icon for menus. And furthermore you might expect it to hold options somewhat less common than the primary nav, unless you’re on mobile. What Discourse has is a sidebar and, when used at desktop resolutions, it’s a normal nav sidebar, not a menu. Collapsing it can be desirable for some users, but while I get that the hamburger-type icon is in some way reminiscent of the structure of the sidebar, it communicates something else by popular convention (and, in some cases, literal design guidelines from big companies).
I know I’m commenting on an old topic here, but I’m back to some Discourse work and noticing some of these UI and UX oddities that stand out to me. It frankly surprises me that this was adopted as the default and remains the icon for this function. It may seem minor but when an app or platform defies widespread design conventions it confuses people, and that affects their daily experience of that app or platform. I still click that button at least once a day expecting it to drop down options or something. I work on a 4k screen so I have no actual need to collapse the sidebar, well… ever. It’s a sort of muscle memory from so many other apps and websites.
Other tools have solved the expand/collapse sidebar behavior with a variety of icons, all of them clearer to me than this one. It’s worth reconsidering IMO.