Over the last several years, I have noticed that most schuls in the US do not recite the pasuk of שמע ישרﭏ at the end of Shirat haYam in Pesukei deZimrah, while every Nusach Ashkenaz siddur I've seen (Ashkenaz, Polin, Anglia, Amerika, E"Y, Lita) still contains it.*§
Why did Ashkenazi schuls in the US stop reciting this passuk in the context of the paragraph of כי לה׳ המלוכה at the end of Shirat haYam ?
*Older siddurim contain it in the same type and setting as the remainder of the paragraph, although some newer ones have shifted to smaller type (see the older Koren [1980s-2007], for a particularly strong example of this phenomenon).
§Siddurim surveyed include the ArtScroll, Koren, Shilo, and some Hebrew Publishing Company siddurim, as well as Rödelheim, Sulzbach, older Birnbaums, Singer, Bloch (Siddur Cha'areh Tefillah) and Machon Moreshes Ashkenaz editions.