The Gemara quotes baraisa that says:

If one goes North from Acco to Keziv everything on his right is tamei with regard to the laws of the Land on the nations (i.e. they don't mark all graves) and it is not subject to the laws of Masser and the sabbatical year.

The gemara questions how Keziv could be the north point of EY if we learned in our Mishna that Acco is the north most point (at least as far as Gittim are concerned.) We answer that Acco is indeed the north most point but there is a thin strip which includes the west half of Acco and runs north to Keziv which is EY.

What I'm not understanding is that if we draw a line from Acco North to Keziv (Artscroll thinks Acco is present day Acco and Keziv is Achzib, I guess they mean Tel Achzib, which is 8 miles north of Acco) and look to the east of it we find Tzfat!

So I'm confused how this northern border looks. Certainly Tzfat is part of Eretz Yisrael. Perhaps the border runs like a V? But the brarisa says EVERYTHING to the East is not Eretz Yisrael....

do ppl in israel who are not within those ey boundaries keep the minhagim of EY?

For example, yes, people in Eilat must keep two days of Yom Tov.

Thank you for the definitive psak. I wonder why R. Moshe Shternbuch in Teshuvot Ve-hanhagot 4:116 would suggest it's possible otherwise.

You're welcome.

R Elishuv says you have to keep 2 days (or so I heard quoted in his name...)

The problem is not only in Eilat, by the way, but so too even in Yerushalayim - R. Shternbuch writes that even in Yerushalayim the Maran Ha-Gaon Me-Brisk would hold two days [at least as far a melachot d'orayta] because his neighborhood did not have a mesorah of habitation since BHM times.