If the person lying by omission has the intent of causing the recipient to have false understanding (e.g. to provide basis for a choice) then it’s at least as dishonest as a blatant lie.

If they omit information because they think it’s irrelevant then it’s just ignorance or negligence. Dishonest in the sense that it’s not forthright.

Neither lies nor omission are inherently unethical without some basis for the relation between the two people being honesty/loyalty.