> There are simple ethical alternatives, just not disclose that information, or just avoid that questions.
That's not always possible.
> Lying in this case is clearly unacceptable.
In what case?
If you're really taking the stance that anonymity is never ethically more important than honesty, that's a pretty extreme stance. Are women being stalked by an ex who is a cop required to be honest about their identities? Are reporters investigating sex trafficking required to be honest about their identities?
In a more broad sense, there are many corporations and governments out there who follow no ethical rules whatsoever and are sucking up as much information on everyone as they can. These people don't have any sort of right to that information, and don't have a right to my honesty. If anything, I think the moral thing to do, if any, is to hamper these people's efforts by feeding them as much incorrect information as much as possible.
> It is also a signalling issue.
Everything is a signaling issue. Your post, for example, signals you as a person who has not-very-nuanced opinions about nuanced topics, and therefore can't be trusted in nuanced situations. If you wouldn't lie to the police to protect a pot smoker, or lie to an advertiser to protect my privacy, you aren't an ethical person by my standards. I do believe you have good intentions, but intentions that don't translate into the correct actions don't mean much.
> There is a universal social contract not to lie.
This is what I mean about not-very-nuanced opinions.
> If someone is willing to lie in such minor issues, then such person is totally untrustworthy, because there is no reason to assume they would not lie or hold other social contracts in more serious cases when it does not suit them.
1. What issues are you considering minor? You seem extraordinarily willing to generalize some specific-but-not-described situation you're imagining to all of reality.
2. There are lots of reasons to assume that someone who lies to cops won't lie to me, for example.