"Time Machine's oldest backups are deleted when disk becomes full"

This has been the case since Time Machine was released 13 years ago.

This is why I always recommend:

1. Go to Apple menu - System Preferences -> Time Machine and turn off Time Machine, but enable the menubar icon for it.
2. Use the menubar icon for it to manually backup after you have verified you have copied any archives you aren't keeping on your computer are copied somewhere else other than your current Time Machine drive. And only run Time Machine when you aren't actually running the machine for production purposes (i.e. when sleeping and not running long processes).
3. Use Carbon Copy Cloner to copy whole folders to other drives from http://www.bombich.com/ if you want to ensure the integrity of those archives.

Keep in mind Time machine backups function based on recursive storing and deletion when full, starting with hourly, then daily, then weekly, then monthly, and annual backups. Any data not on your original drive that gets removed will potentially get erased from your Time Machine backup. So don't depend on it alone to store anything.