Only archived tasks can be deleted.
Open the view task modal

Click “❌ Delete”

Confirm by clicking “Delete task”

You will see a confirmation the task has been deleted

No notifications are sent to task watchers that has has been deleted. Only a mention about it in the Primary Message Channel can be ephemerally visible.

Only an assignee, or a task creator, can delete a task.
It’s impossible. Tasks are deleted from the database and it’s not possible to retrieve them.