use action wp_trash_post (https://core.trac.wordpress.org/browser/tags/4.5.3/src/wp-includes/post.php#L2656) to perform lookups before trashing

Also check into untrash actions

course

• remove prerequisites from any other courses
• remove from membership auto-enrollment
• cancel enrollments?
• delete child sections
• remove access plans (on deletion) (via Remove related access plans on course/membership deletion #1518, Release: Version 4.15.0 #1522)

sections

• unset lesson parent meta data

lesson

• remove prerequisites from any other lessons
• warnings on builder when lessons deleted (empty node on syllabus)

quiz

• remove from any lessons
• remove quiz attempts

questions

• remove from any quizzes

track

• remove prerequisite from any courses

membership

• remove restriction settings from any post which supports membership restrictions
• remove from members-only access plans
• check and remove from site wide restriction setting
• cancel enrollments?
• remove access plans (on deletion) (via Remove related access plans on course/membership deletion #1518, Release: Version 4.15.0 #1522)

user

• postmeta cleanup
• order anonymization
• cancel recurring orders

User cleanup moved to #940

orders

• related transactions

WIP: https://github.com/gocodebox/lifterlms/blob/master/includes/class.llms.post.relationships.php