Between 1993 and 1999. The African village and bonobo island opened in 1993, and the zebra crossing between the large parking lot and the park is still present. The connecting tunnel under the Leuvensesteenweg replacing it opened in 1999.
It seems to be the English version of the Dutch map for 1998 on the Les Zoos dans le Monde website's Planckendael page. 1998 was the first year Planckendael displayed koalas, and there is an area labeled Australian project on these maps.
Thanks very much @KevinB for narrowing down the timeline. This is actually a great map, as it's very large when it folds out and the attention to detail is fantastic. So many zoos around the world are phasing out paper maps and yet those same zoos used to produce artistic masterpieces.