Does this remain true for DuckDuckGo? I was under the impression that they used to be that way, but that they had switched over to using their own search backend some time ago.
ddg doesn't have date filtering search syntax like google has, for example "before:2018-12-31", they offer only a few date filtering options shown on a combo.
First, they do not hide that they aggregate. It's fanboys that tend to overextend DDG's capabilities, not them (at least for best of my knowledge).
And second, given the resources they have, they are doing more than enough, and they do provide a great service. It might not solve the underlying problem of privacy, but at least it helps protecting the privacy of those people using it.
To sum up, I do not think it's fair to bash DDG for aggregating.