As far as we know our plug-ins work well on High Sierra for all the formats: VST, AAX and AU. The only problem is garageband. The reason for this would seem to be that Apple has deliberately decided that garageband should no longer be able to use normal AUs. Garageband is now an iPhone App first and a computer program second. There may still be workarounds. You could try this one:
Still, this is a short term solution. It may be that you have to use another DAW than garageband if you want to use Toontrack products in the future.