The Ekayana Shakha is a lost shakha of the Shukla Yajur Veda.
The Sri Vaishnava Pancharatra Agamas are based on the Ekayana shakha.
Yamunacharya in his work "Agama Pramanya", written in the 10th or 11th century, has said that the Ekayana shakha was still alive in his time and people studied it like any other shakha.
So, it must have been lost sometime between now and then.
However, even though the text is lost, are there people alive today who claim that that is their ancestral shakha?
Dvijas recollect ancestral gotras, ancestral Vedic shakha, and grihya sutra. For example, most Brahmins in south India follow the Krishna Yajur Veda.
So is there anyone who claims his ancestral shakha is the ekayana shakha?