There are menu items in Zoho Notebook Desktop on MacOS to 'Paste' and 'Paste and Match Style', one of which in any app usually pastes basic/unformatted text. However, both paste text with bold/colour/background etc. (eg. from a web page).

The CMD+Shift+v key combination does nothing; again in any app this usually pastes without formatting.

A workaround is to paste text into the 'Title' field, then cut and paste it into the body.