There have always been plug-ins, especially by small developers on the Mac, which are AU only. Apple Silicon macs can now run iOS / iPadOS AUs as well, so ther

There have always been plug-ins, especially by small developers on the Mac, which are AU only. Apple Silicon macs can now run iOS / iPadOS AUs as well, so there are a ton of plugins from that world that are not available as VSTs. Apple Logic can't load VSTs, so typically on the Mac it makes sense to just have AUs. Ableton Live on the Mac can load VSTs or AU, but there isn't really a reason to keep the VSTs around.