Newsprint definitely works, but opinion is divided on why. In a series of tests stropping was performed on ordinary paper, plain newsprint paper and inked newsprint paper. All had an effect, increasing in that order; ie inked newsprint was best. Clearly then, the paper contributes something but the most dramatic effect is due to the ink.
Depending on where you are in the world and what type of paper you read, the effect will most likely vary. Some modern papers use a type of colour inkjet technology, most papers use pigment in a carrier of vegetable oil (usually a soy derivative) and older formulations of ink had the pigment carried in a petroleum-based (mineral oil) binder.
I think that the main contributing factor is the pigment - most blacks use carbon/graphite compounds and some colours have chromium in them. Carbon is recognised for refining the edge of a razor (I use a graphite coated strop, for instance) and in newspaper ink we are talking of a grit size of around 0.1 microns - very fine indeed, hence the need for a lot of stropping with newspaper!
20 or so years ago it was quite normal for the carrier to be oil - it left a mess on your hands, window cleaners used bunched-up newspapers to impart a final shine on windows and you could use a bit of putty to pull an image off a sheet of newspaper. This oiliness and the way they tried to get around the problem may be the reason why newspaper stropping worked better in the past. 'Driers' were used to reduce the oiliness - a common drier was a powder called litharge, also known as lead oxide. Old US military service issue strops came with a bar of lead - not only did it help in breaking in the strop, like rolling a bottle over it, but it also added something else that improved the edge of the razor. Prior to this in the late 1800s we find references to using a bar of pewter to increase the efficiency of strops - old pewter contained lead as an alloy. Even earlier references mention using sugar of lead (lead acetate) in strop dressings.
I'm not sure what the significance of using lead was, but in an old newspaper with heavy black print you had a combination of the abrasiveness of the paper itself, the abrasiveness of the carbon pigment and the (slip? lubrication? scratch filling property?) of the lead. I suppose the oil carrier was also a bonus - it would serve to coat the blade and help fight against oxidation.
Regards,
Neil