Many users found them confusing because ticket offer same functionality plus a whole lot more. They just seemed redundant in activeCollab Corporate and that resulted in few forum topics where users asked to explain the difference and explain how they can use them.

If you really need them and know how to utilize them you'll install them from Administration > Modules.

Checklist are irreplaceable in activeCollab Small Biz because it does not support tickets and are enabled by default.