Why can I not set Safari keyboard shortcuts? Conflicts with secondary, unlisted Back/Forward commands

It seems like this is a widespread issue. I tried redefining shortcuts such as creating new windows, but although the shortcut appears in the menu bar it doesn't actually work—Safari uses the default extension. This happens for other Safari menu bar items as well.

Sadly this doesn't seem to have been fixed even on Safari 14 (shipped with macOS Big Sur).

In order to overcome this strange limitation/bug, I use BetterTouchTool to set up custom keyboard shortcuts, and it works pretty well.

I can / must confirm your experience. I tried your custom shortcuts in Safari– they ONLY work in tabs without "history".

I tried to google this phenomenon but didn't find much help, either.

Btw, did you notice that these two "stubborn" shortcuts work also with finder windows? So they might be valid system wide and kind of default.

At last I found a reliable source, stating:

• Human Interface Guidelines
  • Keyboard
    • Reserved and Expected Keyboard Shortcuts

The following keyboard shortcuts are either reserved by the system or well-known by users.

Primary Key	Keyboard Shortcut	Action
Left arrow	Command-Left arrow	Change the keyboard layout to current layout of system script.

So CMDLeft arrow is definitely a "reserved shortcut", though with an unexpected "mission"...

(Me: MacOS 13.6, Safari 11.1, PC keyboard)