Reporting options available for Text-Only emails.
There is no available reporting to determine the performance of Text-Only emails. When an email is sent from Marketo, both the HTML and Text-Only versions are sent, and the receiving server chooses the one to hand off.
Marketo uses a single pixel image to track Open activity emails. Since there are no images in Text-Only emails, Open activities cannot be tracked. However, Clicked Link activities can be tracked as long as URLs in the Text-Only emails are formatted according to this document - Add Tracked Links to a Text Email - Marketo Docs - Product Documentation.