Following up to my own post...
First, it seems that note history is a premium feature, I recently updated to premium, which explains why I did not notice the issue before.
It seems that when I search, evernote searches the note history as well. That is search will find notes that include the search terms, if the search terms show up in the history, even if those terms are not in the current version of the note. This is good, although not real obvious as implemented. This is good because if gives the opportunity to search history for confidential information -- so that action can be taken to protect that information. [Update: I was wrong, there doesn't seem to be a way to search history?]
Currently what I'm doing is encrypting the text in my note, and then to be sure the unencrypted version is not available in history, I duplicate the note, then permanently delete the original note (along with its history). I've noticed the history is not copied when a note is duplicated. Tedious but it gets the job done.
Is there a better way ?
Some ideas that might improve usability / security:
* evernote could warn user when the encrypt text that history may contain unencrypted version, and offer user option to delete history.
* evenote could keep history encrypted and password protected.
* (unrelated to original issue) when duplicating a note it might be nice if there was an option to also duplicate the history. I'd leave the default as-is (no history duplication), or prompt every time so user is aware of history situation on the new note.