How to set 'planned for' attribute to 'Unassigned' in RTC ALM
One answer
In the OSLC API, getting the work item, removing the entire entry for the planned for attribute value and PUTing the new payload - including the If-Match etag should do the trick.
In some cases such as the category there are "unassigned" values you can set.
I do not know if there is for planned for. You can try to follow the resource shape and try to find if there is a unassigned value for the planned for attribute. In any case not setting the value as explained above should create the same result.