wgContentTranslationHighMTCategory was added in CX1 to set a category when the translation has machine translation greater than 75%(not configurable). The value of this configuration is a category.
But AFAIK there is no wiki that use this setting. CX2 replace this with section level machine translation usage and with a tracking category. See T190798: CX2: Add to a maintenance category when translation is published with too much unmodified text
So we can consider removing this setting.