[tomsem]

The difference from iOS 14, of course, is that the sidebar toggle button never went away, and now, it does as soon as you select a category. Even in Portrait, the toggle disappears so you have to either retrace breadcrumbs or rotate to landscape and back to get it to go away when you decide not to choose anything.

I would say it is pretty consistent for sidebar to remain in Landscape after selecting something, rather than hiding. Same is true of News, Music, Notes etc. They think with so much horizontal space you might want to have quick access to navigate, I guess.

This is not an issue with Keyboard, where shift+command+L toggles. You might think that should work with other apps (or at least Apple apps) with sidebars. You would be wrong. Control+Command+s is the one that works with News and Music. But it looks like they (reasonably) wanted to match macOS Safari hotkeys.

I barely use Bookmarks (or Safari sidebar generally) at all, so I would never have noticed this.