Is it expected I need to pass the optin marker for all usage when I use OptIn for an actual platform implementation with Kotlin 1.9.20:

// common
expect interface Closeable {
  fun close()
}
// target, here wasmMain
@OptIn(ExperimentalStdlibApi::class)
public actual typealias Closeable = kotlin.AutoCloseable

When I use this interface in common,

class Foo: Closeable

, I get this error:

This declaration needs opt-in

.

Yes, that's expected (see the docs). You can pass the opt-in as a compiler argument to enable it for all your code at once.