How to achieve sub-task in a task using Issue Matrix

This can be achieved using existing functionality of Issue Matrix (IM):

1. Create a separate IM field for sub-tasks.

2. Specify which columns are relevant to sub-tasks.

3. Add the new IM field as a column in the Epic IM field.

4. The sub-tasks of each issue inside the epic are shown on that line as well.

Check out an example on how it looks in the screenshot below:

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Find the below example for reference:

1. Create respective fields in the sub-task custom field as shown in the below screenshot.
   
   

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2. Create respective fields in Epic custom field as shown in the below screenshot.
   
   

    

3. Refer to the below gif for reference.