How can I make a keyboard shortcut to strikethrough text in the Notes app?
It doesn't need any 3rd party solution, you can do this natively.
System Prefs > Keyboard > Shortcuts > App Shortcuts.
Click the + sign, select the Notes app [or all apps], the name of the menu command & add the key command of your choice.
BTW, the native functionality doesn't care where in the menu hierarchy it is, it will find it anyway. The only time you ever need to specify is if there are two identically-named functions in different menus in which case you can force a specific menu by menu name->sub menu->function syntax
