Variable fonts have a proposed new mechanism, a version 2 update to the AVAR table, to allow variable fonts to fence off unusable portions of the designspace, and otherwise distort the mappings. About AVAR2: https://type.today/en/journal/avar
For the bleeding-edge font folks… @lorp has a new tool for experimentally testing and modifying variable font AVAR2 mappings with interactive live visual output. You can drag mappings around AND then export the modified font! Wow.