UA is being unfriendly about this, on purpose so that they can hold onto your money. I think that, yes, you have to cancel by April 30 to get your UA award miles back without paying the redeposit fee. And if you paid cash for that flight and cancel by April 30, then you'd have a voucher (meaning they get to keep your cash) good for two years from the date you cancel, each voucher tied to the passenger's name. If UA cancels the flight, then you can receive a cash refund per DoT rules (but airlines are trying to fight that regulation right now since they are hurting cash-wise). If you don't cancel by April 30, then you're on the hook for any penalties for changes/cancelations unless UA makes the call to extend their own deadline.
I have tickets for my family on UA in July to Europe paid for with URs going through my CSR portal. The first leg is into BCN and what I've seen so far is that Spain is probably going to be closed to international travel until the end of the year. I fully expect UA to cancel my flight, but to count on that is a gamble and I have to decide if I'm willing to take that gamble. If UA doesn't cancel my flight then I'm out over 500K URs.

Or I could change my flight and pay the $450 change fee to use the UA funds (over $7900 worth) for travel by this December (12 months from original booking). It sucks. Now I'm wishing that I'd transferred over URs to UA to purchase the tickets instead of using URs. It was just so much less to pay with the 1.5x URs back when I made my travel plans.