Switch between "In Library" and "All Apple Music" in iTunes Search
If you enable All Controls in System Preferences → Keyboard → Shortcuts, then you can ⇥tab to the button and use space to select it. There does not seem to be a keyboard shortcut for toggling the option directly.
On a US keyboard, you can use Command + Shift + [ and Command + Shift + ] to cycle among the tabs.
Here's an overview of keyboard shortcuts for iTunes.