Y14.5.1-2019 goes into great detail on single solution datums, including the constrained L2 math. Y14.5-2018 made a subtle change from previous revisions such that the new default method for stabilization is a single solution datum instead of the previous practice of the candidate datum set. Y14.5-2018 does not explicitly state "constrained L2", that is in Y14.5.1-2019.
Y14.5-2018 Para. 7.11.2 discusses the new default requirement being a single solution and refers you to Y14.5.1.
