Charles Ellicott Commentary Deuteronomy 19:14

"Thou shalt not remove thy neighbor`s landmark, which they of old time have set, in thine inheritance which thou shalt inherit, in the land that Jehovah thy God giveth thee to possess it." — Deuteronomy 19:14 (ASV)

You shall not remove your neighbour's landmark. —Another law manifestly appropriate here, where it appears for the first time, like the “field” in the tenth commandment (Deuteronomy 5:21). But the immediate connection is not obvious. Perhaps the idea is to caution the people to avoid a most certain incentive to hatred and murder. Ancient landmarks are also important and almost sacred witnesses.

They of former times. —The first dividers of the land. There is no idea of antiquity about the expression.