I'm as baffled as the user!
So let us get this straight. This is an application that mimics post-it notes. Post-it notes are also used very often as to-do lists. I'm sure Microsoft is aware of that: people use Sticky Notes as to-do lists, quick scribble. I don't want to use paper. I want to be more ecofriendly, jolt it down, type it quickly on a sticky note.
If there's any place where strikethrough is important, it would be here. Why remove the hotkey? It's nonsensical. It's not even that we are asking for a feature addition. Why was it removed? Probably the style that's applied the most and you've decide to take it the hotkey off. Now im having to use my mouse everytime this is required.
Terrible design choice.