I do know that it is expected for the task information to be removed when tasks are removed in the type of scenario you are describing. There isn't really a way to force the feed to retain that information (from a user perspective). This unfortunately would need to become an enhancement request to ask for the info to be retained.
If the info would need to be documented/retained, currently I would recommend before the deletion of the tasks were to be performed, add one Feed comment which sums up the work done already and the reasoning for removing the task template. This allows the info to be kept in one single post which is unrelated to the template or tasks so it is not removed by the deletion.
Would something like that help?