deleting a line of text caused the text to hightlight and strikethrough but the text is not removed from the document.
How can I undo this setting?
You are in “track changes mode”. To exit from this mode, Edit>Track Changes>Record (this will remove the check mark).
It was activated by a spurious use of the Shift key while copying some text (Ctrl+Shift+C instead of Ctrl+C).
To show the community your question has been answered, click the ✓ next to the correct answer, and “upvote” by clicking on the ^ arrow of any helpful answers. These are the mechanisms for communicating the quality of the Q&A on this site. Thanks!
In case you need clarification, edit your question (not an answer) or comment the relevant answer.