In the Wiki description, Sonthert/Lanth (SM 1918) states:
In the Abyss Rift article, it states that the apocryphal focal point of anomalies in the Rift is adjacent to three Red-Zoned worlds, one of which is Sonthert/Lanth (the other two are Victoria/Lanth and Ylaven/Lanth).
The UWP in the Wiki description shows it as Amber, as does Travellermap.
I'm assuming the zone change was part of reconciling the UWP with T5SS standards.
In S3 (The Spinward Marches), it's listed as having a Red Zone.Sonthert is interdicted both to protect its human population from interference, and to protect the rest of the Imperium from the megalomaniac "Emperor of All Things" and his armored knights, who have declared their intent to conquer the universe, starting with anyone foolish enough to land on Sonthert.
In the Abyss Rift article, it states that the apocryphal focal point of anomalies in the Rift is adjacent to three Red-Zoned worlds, one of which is Sonthert/Lanth (the other two are Victoria/Lanth and Ylaven/Lanth).
The UWP in the Wiki description shows it as Amber, as does Travellermap.
I'm assuming the zone change was part of reconciling the UWP with T5SS standards.