is it intended that

KotlinMultiplatformExtension.compilerOptions

is

internal

now in 1.9.20-RC(2)?

Yeah, apparently there were some issues with it that still need to be resolved. https://kotlinlang.slack.com/archives/C0KLZSCHF/p1697220966633699?thread_ts=1697219629.400059&cid=C0KLZSCHF