When you are in an Inititive window, and select Add in the Releases section, you can add that various releases. If you look at the Release, it shows that it "Belongs to" the Initiative. But, there's no way to create the same releationship from the Release. If you click Add next to Linked Records in the Release, there are multiple relationship options but not Belongs to so the Release ends up in a different section of the Initiative relationships.
Also, when you are adding Releases in the Initiative window, there doesn't seem to be a way to add released Releases. There are times where we need multiple Releases to complete an Initiative and if they both didn't get added before they are released, there doesn't seem to be a way to add it so the progress bar is accurate.
Thank you for the idea. Both of these can be done currently using the standard Initiative field on the release details. If you select an initiative in this field, the Belongs to record link will be added automatically. You can also add new initiatives to shipped releases using this field.