Actually, it is quite easy. Just follow these steps (on the PC version 3.5, at least):
- Click on Tools
- Click on Customize…
- Click on the Keyboard tab at the top
- To the RIGHT of the Shortcut Keys box, click on LibreOffice
- Below the Shortcut Keys box, find the Functions box, then find and click on Edit
- In the next box, the Function box, find and click on Paste Unformatted Text
- In the Shortcut Keys box above, find and click once on Ctrl+V
- On the RIGHT, click the button titled Modify or (in newer libreoffice version Assign)
- Optional: Click the Save… button and save your keyboard configuration so you can import in into another computer or if you reinstall LibreOffice
- Click the OK button at the bottom to finish.
Source: https://ask.libreoffice.org/t/paste-unformatted-text-default-option/336