We want to keep it this way so people can still make plugins work for older versions of Cura, like 3.6, where the 'api' field with just a single major version, instead of the 'supported_sdk_versions', which can be either a version-string like '7.6.0' or a list of version strings.
The Cura 4.9 release will have expanded functionality. If you have a plug-in that uses this functionality, marking it as using SDK 7.5.0 will notify older Cura releases that they can't use that plug-in.
We're changing the unit of one of our settings, so we need to implement a version upgrade to change that for our users in case they changed this setting themselves.
Contributes to issue CURA-7118.
