While it doesn’t solve the issue of not knowing a feature is there in the first place, if there’s something you’d like to do in an app, you can get a long way by hitting “Help” in the menu bar and typing it into the search field. If there are any matches for it you’ll immediately know you can do it, and it will open up the menu and show you exactly where it is (and the keyboard shortcut, if there is one).

This is the primary way I look for power-user or other non-obvious features in new Mac apps.

Menus are still superior to help search for feature discovery because they are an explicit navigable enumeration of the feature set surface; if help doesn't have the right synonyms, you'll be stuck, whereas menu item listing might hint at different ways to accomplish the same end goal.

FYI the search box in the help menu searches the rest of the menu bar. It's great for feature discovery if you think something might exist but don't know where it is.

Mac menus list the keyboard shortcuts for menu items right next to the items.

So do menus on all desktop platforms I've used.

My point is that menus work as a way of exposing functionality and making it discoverable through enumeration in a way that contextual UIs and fashionable techniques of the day don't.

I'm pretty sure that visible shortcuts are a CUA thing. They date way back - even text-mode menu-driven DOS apps had them, e.g. all the Borland IDEs.

Is that a macOS thing, or just a conceptual problem with keyboard shortcuts?

IMHO macOS does a pretty decent job of discoverability for shortcuts as they’ll be listed there in the menu.

edit: originally I doubted whether Windows does this or not. It does! Somehow I missed this despite using Windows every day...

Windows programs absolutely do show shortcut keys in their menus.

http://homepage.eircom.net/~pcengineers/kbguide.html

This is in addition to access keys (also explained in the document above), underlined letters you can type to instantly navigate the menus and select even the items for which no shortcut is defined.

My bad.

Here's what I get on Windows 10 in Firefox, showing both the shortcuts and the menu keys. https://imgur.com/a/EUEr8y5