What characterizes the short toledoth at Gen 11:10-26?

Also, what is its function? Was it really necessary? What do the genealogy and years lived imply about the faith of Abram? The toledoth recounts “the generations of Shem”—the originator of the Semites—and we must bear in mind that it is not first and foremost about Shem himself but his family, or descendants. So this connects Shem, Noah’s son, with Abram, the focus of the next great narrative. Noah was blessed, and he passed on that blessing explicitly to Shem (Gen 9:26-27) and his line. The promise of Shem, and by extension the promise of the “seed” (3:15), passed on through this line ultimately to Abram. That is the main point. The genealogy also illustrates two facts, one already announced—that the span of man’s life would be 120 years (6:3)—and one more obscure—that the first five generations from Noah were alive, if not on hand, even into the time of Abram. This further entails that a living memory survived to Abram’s time not only of the Lord, but of the sacrifice and God’s covenant, of the Flood and its reason, and of evil, antediluvian days. Unless the families were utterly separated, which is quite possible, or lost touch with the old faith, which is also possible, the memory of the Lord and of sacrifice would not have to be wholly renewed in Abram.

Is it not remarkable that Gen 5 and 11 show ten generations each? And are there not eleven generations listed in Luke 3?

From Adam to Noah (inclusive) was ten generations, while from Shem to Abram (also inclusive) was ten generations. Does this mean there was some contrivance? Not necessarily; such an isolated detail could easily have been a coincidence, or God could have arranged this as some sort of subtle lesson, though I confess I do not know what that would be. It could also, or instead, mean there was selection by an author or redactor of Genesis; in fact, there are eleven generations listed from Shem to Abraham in the genealogy of Luke 3:34-36. So, one might argue, perhaps there was a deliberate omission, and creating two sets of ten names seems a plausible explanation of the omission. The problem is that the earliest versions of Septuagint lack the name “Cainan” (as Genesis 11 lacks it), while some later versions added it. That would explain how Luke got it in his copy. Still, does this not represent an error in the inerrant text of Luke? Well, no, and here things get even more interesting. As the great Baptist theology John Gill has it, “This Cainan is not mentioned by Moses in Gen 11:12 nor has he ever appeared in any Hebrew copy of the Old Testament, nor in the Samaritan version, nor in the Targum; nor is he mentioned by Josephus, nor in 1 Chron 1:24 where the genealogy is repeated; nor is it in Beza’s most ancient Greek copy of Luke: it indeed stands in the present copies of the Septuagint, but was not originally there”. In other words, the name “Cainan” was added to the text of Luke by copyists who were relying on erroneous copies of the Septuagint to “correct” the text of Luke. So the originals of both Gen 11 and Luke 3 probably had ten names apiece. As to the whole issue of contrivance, see entries above that mention this concept; there is no new challenge here.