There appear to be two main theories at the moment:
Saying things that are not true with the understanding that the person you are speaking is likely to think (that you think that) what you’re saying is true is always bad.
Speaking in such a way is an act of violence, and is acceptable only in circumstances analogous to when violence is appropriate.
Theory 1) says that such speaking is directly opposed to truth and thus intrinsically evil. People who hold this often say that deception by withholding is permissible. (“I don’t know where any Jews are” by which you mean that while you know there are Jews in your house, you aren’t certain exactly where in your house they are - they may be huddled in the back left corner of your secret room instead of the back right corner).
Occasionally people defend theory 1) by saying that lying is always sinful, but that in some cases (Nazis) it is ok-ish, or not that bad. This doesn’t work - sin is never ok, ish or otherwise. If it is in the least sinful, then doing so to save lives is absolutely prohibited, whether we would go to hell for doing so or not. One cannot do evil, however minor, so that good may come of it, however major. For the most part, this is recognized by proponents of 1), though there are those who admit that under the pressure that they might end up lying anyway, but also admit that such would be a worse choice than an attempt at deception - even if from the tone of the interrogation it is clear that the attempt at deception will fail, whereas the lie may not.
Theory 2) is defended in various ways, but mostly by making a distinction between sinful lying and telling an untruth along the lines of the murder/killing distinction. Usually this distinction has something to do with the idea of whether the person you are talking to a) has a right to the truth you wish to conceal, and (I think, though I haven’t heard it explicitly stated as offten as a) ) b) has a right to have what they are told be true. (Analogously, a) does a person have a right to the un-poisoned food that you have, and b) does he have the right to have any food that you do give him (whether he has a right to it or not) be unpoisoned.)
Sometimes methods of defending theory 2 appear to leave open the morality of lying in cases where it shouldn’t be. To me, these mostly appear to be the arguments that ignore b).
Santa Claus etc is sometimes classified as myth - a less than entirely direct way of bringing people to a greater truth that they might otherwise miss. I’m pretty sure there are people in both groups who think that it is both ok and not ok.
I personally hold theory 2 (as was probably obvious - hopefully those who favor 1 will chime in, I’m sure my disbelief of their arguments taints my presentation of them). I tend to think Santa is fine, but that “Barney isn’t on” probably isn’t. (“No those pants don’t make your butt look big,” is, I think, both acceptable and indeed the only correct response.)