<<ATC should give the instruction before the landing crosses the threshold.>>
I presume that a new instruction as I never heard it before. Is there dispensation for a displaced threshold?
Definitely not new, and it is a daily concern for those of us not fortunate enough to have parallel strips of tarmac in segregated mode. The MATS Pt 1 take on it is that "a landing aircraft shall not be permitted to cross the beginning of the runway on its final approach until a preceding aircraft, departing from the same runway, is airborne".