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"Save As..." is available in all apps with a File>Duplication command. If you hold down the option key, "Duplicate" turn into "Save As..."

macOS has never let users edit or add menus items.
 
macOS has never let users edit or add menus items.
As far as I can remember, you have always been able to edit the keyboard shortcuts for menu items.

The OP is just confused because the newer System Preferences Settings is designed awfully and it's very hard to actually do so thanks to you-know-who (Mr. Death)
 
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