Currently, Microsoft did not build in a possibility to assign a shortcut to the paste options separately.
But you can do the following:
Keyboard combinations:
Context Menu Key + T
- If you don’t have the Context Menu key on the keyboard, use Shift + F10 instead. The whole combination will become:
Shift + F10 + T - Another way is to use the keyboard to access the Home menu on the ribbon (H), then the Paste menu (V), then the Text without Format option (T). You do this while holding the (Alt) key:
Hold Alt + H, then V, then T
Resources: keyboard shortcuts – How to paste unformatted text only in OneNote 2010? – Super User
