I track student scores in ScoreCenter. What happens to the gradebook data if the lesson is deleted or my SoftChalk Cloud account expires?

Even if the lesson is removed or your SoftChalk Cloud account expires, this will have no impact on the LMS gradebook.

The LMS grade from each student will remain in the LMS gradebook for as long as that course exists.  Once a score is passed to the LMS gradebook by SoftChalk the grade will remain in the LMS gradebook

ScoreCenter will keep the ScoreCenter attempt data available for at least one year from the date of the attempt. However, once a lesson is deleted or the account expires, the instructor-facing ScoreCenter pages will no longer be available.