Done.
From the looks of it TH759 does not extend as far as I had it mapped either. Per my notes from 2016, "Unnumbered on Vegagerdin map, assumed to end at end of road."
Well it's on the Vegagerðin map now so there's an official source for its endpoint: doesn't go past Stekkjarflatir.
That said, this change impacts exactly zero users. You're about to be the first to claim
any of TH759.