Up until roughly 5 or so years ago, every traveler entering the US was required to turn in a paper declaration form when passing through US Customs. This form asked whether you were carrying various things of interest to customs (such as food, large amounts of currency, etc.).
The paper form was no longer required as of about 5 years ago, but you usually filled out a similar form electronically, and customs officers asked everyone leaving the customs area what they were carrying.
This last time I entered the US, at no point did a US CBP officer ask what I was carrying into the country, nor was I presented with any form, paper or electronic, to fill out.
Does this mean that active declaration to US Customs (traveler initiated conversation in the form of approaching a customs officer and making a declaration) is now required, as it was seemingly not in the past?
I assume the answer is yes, an active declaration is now required (just on the general principle that ignorance of the law is not a defense for not following it). However, a counterpoint is that according to this website, "In Matter of Areguillin, 17 I&N Dec. 308 (BIA 1980), the Board of Immigration Appeals (Board) took the position that an alien can satisfy her burden of proving that she was “inspected and admitted” if she is waived through a border, even if the alien did not actually have any lawful entry documents." So at least for immigration purposes, in some circumstances, one could say the onus is on the CBP to inspect, and all the traveler need do is cooperate with the inspection, rather than actively volunteer information.
I'm somewhat surprised at the lack of at least some signage soliciting customs declarations.
Note for readers familiar with customs in European and other countries: the exit of US Customs is not split into a "green lane" (nothing to declare) and a "red lane" (something to declare), so at no point do travelers exiting US Customs implicitly assert that they have nothing to declare.