"Purify HTML" on plain text fields
The reason there is a "Purify HTML?" setting only for Rich Text fields is that Twig will automatically escape the output of every other field by default, mitigating XSS attack vectors.

Rich Text fields, by nature, are designed to store HTML, therefore their output isn't escaped and you have to rely on things like "Purify HTML?" (which uses http://htmlpurifier.org/) to try and remove anything malicious looking.

If you're plugin needs to output data for a Plain Text field in a way that doesn't get escaped (for whatever reason), you can always run HTMLPurifier on it manually.