I am unable to assign a feature to a release that happened a while ago, as this release doesn't exist in the drop-down.
The same happens with user stories/iterations, etc...
How can I assign a feature or a work item to an old timebox?
What is the reason for limiting the drop-down list to only recent timeboxes?
Check the state field on the timebox you're trying to add items to. Timeboxes in a closed state (Accepted) don't appear in the drop-down list so you can't add items to them. If there wasn't this constraint, the drop-down list would get very long and unmanageable. Just change the state to something that is not considered closed and you should be able to add work to it.