Last I checked, none of the official / built-in Hubitat drivers were using this attribute yet and SmartThings still doesn’t support it so the voter base is probably limited to people with custom drivers that use this attribute (which is also probably pretty small considering the attribute is fairly new and not widely used).
The primary requests we’ve seen around Fan Speeds has been for the Lutron Caseta Fan and the Bond fan integrations… so I would bet that if those got official support for supportedFanSpeeds, this topic would see more interest!