When viewing the Task Detail page in Web Plan Manager, feed items are only able to be added if the plan is checked in. However, if the plan is checked out, users are unable to provide updates/comments directly from the Task Detail page. The reason for this is to avoid confusion when looking at the feed for a task.
When you check out a plan, you are able to edit all of the tasks in the plan before saving. At the time of saving, the system automatically generates feed items for updated tasks. Due to this, we assume all feed entries should be generated by the plan changes themselves when a plan is checked out. When the plan is checked in, you should be able to update/comment on each task individually.
I hope this helps to explain why we do not support manual task feed entry when a plan is checked out. Please let us know if you would like additional clarification or there is anything else we can do for you.