Is it lying if you "lie by omission"?

YES!

Merriam-Webster's Dictionary defines "lie":

1 : to make an untrue statement with intent to deceive
2 : to create a false or misleading impression

By leaving out pieces of a true statement(s), that creates a false or misleading impression. It's like putting together a puzzle and stating that the picture is complete when there's only one tiny little piece missing.

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