As it is possible to support multiple java versions in a library with gradle variants, it is possible to do the same but with Kotlin? Is there any sample project about them?

If you control the supplier and the consumer, sure, as you can then just define a custom attribute that works like the one for Java, set it on the supplier and request it on the consumer. For a more standardized approach you have to make the Kotlin folks define a standard attribute for the Kotlin ecosystem that they then also set and request accordingly if new enough Kotlin Gradle Plugin is used.

to make the Kotlin folks define a standard attribute

Looks like I am forced to use the KSP approach for now then (attaching the Kotlin version to the version of my project)

Currently afair they only have the platform type to have one version for jvm and one for android