Internal

Procedure

Thunderbird Cleanup on Mac
iCloud Contacts Cleanup
iCloud Calendars Cleanup
iCloud Drive Cleanup
Remove internet accounts. Go go System Preferences → Internet Accounts and remove all external internet accounts.
Remove private keys from .ssh.
Log out from and remove Spotify.
Deregister the Mac from Kindle
Log out from iMessage and remove the message database from /Users/<username>/Library/Messages
Remove "Downloads" files.
Safari: Remove passwords: Preferences → Passwords → Remove all. Remove cached website data: Preferences → Websites → Remove all. Safari → Clear history → All history.
Chrome: log out from the google account and Turn off sync and personalization. Clear browsing data.
Log out from and remove Kindle.