Present tense of "unassigned"?
The premise of this question is based on a couple of false assumptions.
As a verb
The present tense of the verb unassigned is unassign. It's not as common, but it's perfectly reasonable to apply the un- prefix to some verbs in order to form their opposite.
From "Assign and unassign" at Pearson:
Assign and unassign
You can unassign an assignment and later re-assign it if you want. After the assignment has been unassigned, the green check mark is removed from the Assigned column in the Homework/Test Manager, and the assignment is no longer posted for your students to see; however, results for any work done in the assignment are preserved.
You can also change the assignment status of multiple assignments at one time.
To unassign an assignment:
- Go to the Homework/Test Manager.
- Select Unassign from the Actions dropdown list for an assignment.
- Click OK in the pop-up window when prompted to confirm. The green checkmark is removed from the Assign column for that assignment.
You can later re-assign the assignment by selecting Assign from the Actions dropdown list.
As an adjective
The present tense of verbs are not normally used adjectivally in the first place. This applies to any verb, whether it's used in the affirmative or the negative.
The following are examples of -ed verbs used adjectivally. Consider each as a complete sentence rather than just a sentence fragment:
✔ She is liked.
✔ She is disliked.
✘ She is like.
✘ She is dislike.✔ He is supported.
✔ He is unsupported.
✘ He is support.
✘ He is unsupport.✔ The homework is assigned.
✔ The homework is unassigned.
✘ The homework is assign.
✘ The homework is unassign.
It's not that there is no negative version of the present-tense adjectival use of assign, but that there is normally no present-tense adjectival use of any verb, assign or otherwise.
In short, unassign is a proper word. It's just that it's not appropriate in some contexts—in the same way that assign is not appropriate in some contexts.