It is a myth.
Instead, (approx $80). Install Lightroom Classic normally on your Mac (it has to live on your internal drive). But store only the Catalog and Photos on the external SSD. lightroom portable mac
Let me guess. You have a MacBook Air with a tiny 256GB hard drive. You love Adobe Lightroom Classic, but that catalog file is 30GB, the previews are 50GB, and your startup disk is screaming red. It is a myth