Salesforce provides the Task Status: Completed.

If you want to differentiate between completed and canceled tasks, set up the status as below.


Use

If you now select the status Cancelled, the system does not enforce the completion of the Micro Tasks.

Also when you delete a Checklist Instance, you can select to Cancel the tasks (vs. deleting them).


For recurring Checklists, you can optionally cancel open tasks when you Add new Checklist Tasks.


Setup

1) Set up the Canceled Status


> Setup > Object Manager -- select Task


Add a Closed Status (here using the name 'Canceled'):



2) Define the Cancelled Status use


> Setup > Custom Metadata Types -- Manage Records for Checklist Settings or create New Custom Metadata Type


Define a record or create New (only first is used)



Set the Canceled status defined earlier (hint: Canceled is US English - Cancelled is GB - just use it consistently ;-)