A legal precedent that is most closely related to your question is Matter of Cantu, 17 I&N Dec. 190 (BIA 1978).
In 1906, the Rio Grande was artificially diverted north, which cut off a part of Texas from the rest of the state. During this period, Mexico, and not the United States, exercised de facto jurisdiction over this area, but legally, it remained United States territory. (In 1970, it was ceded to Mexico by treaty, resolving the unusual situation.) In 1978, the Board of Immigration Appeals had to determine the citizenship of a man who was born in the area in 1935.
The majority distinguished between an actual renunciation of US jurisdiction over the land (which occurred in 1970) and a situation where the US had determined that it still had jurisdiction but simply chosen not to exercise it for some reason (speculated to be "because of fear of causing an international incident or disturbing international harmony"), which was the case prior to 1970. In the latter situation, US citizenship still attaches at birth due to the Fourteenth Amendment (although to pedantic, the Board merely refused to allow Cantu to be deported, and did not technically declare him to be a citizen).
A wall exists along some parts of the US-Mexico border, and not others. This question could have been asked about the parts that don't have a wall. The United States can't monitor all of the border, all the time; many people have succeeded in crossing into the United States between ports of entry. Would you imagine that, if they gave birth in an area that isn't regularly patrolled by CBP, it means the baby isn't a US citizen?
The wall really doesn't change anything. CBP could, if they saw fit, patrol the area south of the wall that is still part of the United States. The fact that that part isn't blocked off doesn't mean that it's legal for non-citizens to simply go there without being inspected by CBP. Even if one views the erection of the wall at a particular latitude as an implied decision not to exercise jurisdiction south of it, the Cantu precedent is that nothing about US jurisdiction for citizenship purposes is actually changed by such a decision.