Itzick Shai has made available his 2011 article, האם התקיים מנהג המילה בפלשת בתקופת הברזל [“Was Circumcision practiced in Philistia in the Iron Age II?”] (Eretz-Israel 30: 413-18), on Academia.org.
In it, Shai responds to Avraham Faust’s contention, in Israel’s Ethnogenesis (2006: 147-48), that Philistine non-circumcision was confined to Iron Age I. Faust notes David’s description of Goliath as an “uncircumcised Philistine” in 1 Sam 17:26, 36, in which the term “uncircumcised” is intended as an insult. Faust claims that this type of insult is only seen in texts referring to Iron Age I.
But Shai discusses two biblical texts which suggest that Faust’s conclusion is incorrect, and also discusses the finding of erect penis pottery at Philistine Ashkelon and Gath which appear to reflect uncircumcised penises. Shai points out the absence, in Jeremiah 9:24-25, of the Philistines from the list of circumcised peoples in that passage. Also, in Ezekiel 32:29-32, the Philistines are not among the nations who are punished by being made to lie down in the netherworld with “the uncircumcised”.
For these, and other reasons, Shai concludes that Philistine non-circumcision continued to be a distinct ethnic marker (or more to the point, a lack of a mark) well into Iron Age II.
So David’s insult of “uncircumcised Philistine” could well be nothing more than a literary embellishment in the composition of 1 Samuel 17. That is, it probably does not imply any historical memory of Goliath’s turtleneck.