Correct me if I'm wrong, but a lie of omission is when you intentionally leave out information when dealing with a subject. Not telling someone everything you know isn't the same thing.
I would say it's deliberately not telling someone something they should/need to/deserve to know. Which is what he admitted to in Civil War.
ETA: It doesn't really matter either way, though, unless you honestly believe Steve never talked to Tony about Barnes/Winter Soldier at any point between TWS and Civil War, even though they worked together throughout AOU during which time Steve openly had Falcon running side missions out of Avengers HQ specifically to find Bucky.
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