Chrome and Firefox ⌘⌥↑ and ⌘⌥↓work fine.
Safari fails me here.
I did check macOS’ keyboard shortcuts, but no apparent conflict there.
Problem persists after reboot.
macOS 11.6
Safari 15.0
According to @mbostock both shortcuts are currently blocked by the Reading List feature in Safari 15.
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Grrr… I see it the Bookmarks menu. No way to delete it… 