Currently it's somewhat complicated. Plugins for IDEA 2020.2 and older are released from classic repo
https://github.com/JetBrains/kotlin. Plugins for IDEA 2020.3 and later are released from new repo
https://github.com/JetBrains/intellij-kotlin. In the future it will probably be just merged into
https://github.com/JetBrains/intellij-community.
The general rule is that all new plugin features will be released for new versions of IDEA only. Certain bugfixes will be backported to older versions. This is the same logic that works for releases of IDEA itself. But also keep in mind that every Kotlin plugin will bundle a certain version of the Kotlin compiler, that will continue to be released from
https://github.com/JetBrains/kotlin. So technically there could be several versions of the Kotlin IDEA 2020.3 plugin, one for each recent compiler version (for example 203-1.4.20 and 203-1.4.30).