Shortcut to open/close sidebar in Notes app?
They don't actually call it a sidebar, they term it 'Folders'

It's in the View menu as Show or Hide Folders, with no default shortcut

You could make your own shortcut in System Prefs > Keyboard > Shortcuts >App Shortcuts - though you'd actually have to make two, for the different wording.
If you chose to re-use  Cmd ⌘   Shift ⇧   L  you would also have to reassign Checklist from the Format menu to something else before it will work.

Shown using a different key combo below for simplicity