Chaser's Public API returns data on all of your tasks, including completed tasks for up to one year since completion date.
1. Authentication
Anywhere in Slack, type:
/chaser get-chaser-webhook-token
The token provided can be used to authenticate requests:

2. Get all tasks
Making an HTTP GET request:
curl -X GET https://slack.chaseforme.com/webapi/tasks -H "Authorization: Bearer PLACE_API_TOKEN_HERE"
Example Response

NOTE
- Information on Slack objects like users and channels can be retrieved from Slack fusing Slack's public API.
- completedAt is the date the task is completed by all assignees. Empty otherwise.
- status can be one of the following "Not Acknowledged", "Acknowledged", "Completed".