Could you please elaborate on how we should proceed further with this suppress?

If this case is no longer supported (or was unsupported but not explicitly reported), shouldn't we have any kind of migration plan?

This suppress is a counterpart for @Suppress("ACTUAL_FUNCTION_WITH_DEFAULT_ARGUMENTS"). You can see that you already have a @Suppress("ACTUAL_FUNCTION_WITH_DEFAULT_ARGUMENTS") inside public actual open class TestBase.

If you didn't have @Suppress("ACTUAL_FUNCTION_WITH_DEFAULT_ARGUMENTS") then NO_ACTUAL_CLASS_MEMBER_FOR_EXPECTED_CLASS wouldn't be reported

Unfortunately, I had to rework the way we report ACTUAL_FUNCTION_WITH_DEFAULT_ARGUMENTS. Before my changes, it was a separate checker. Now it's checked in the expect-actual matcher and it's considered a common expect-actual mismatch incompatibility

And such types of expect-actual mismatch incompatibilities are reported two times:

// Common
expect class Bar {
    fun foo(): Any
}

// Platform
actual class Bar { // NO_ACTUAL_CLASS_MEMBER_FOR_EXPECTED_CLASS (first time)
    actual fun foo(): String { // ACTUAL_WITHOUT_EXPECT (second time)
        return ""
    }
}

That's why you need to suppress ACTUAL_FUNCTION_WITH_DEFAULT_ARGUMENTS two times (But they are two separate diagnostics)

Probably it will be more clear If I change Counterpart for ACTUAL_FUNCTION_WITH_DEFAULT_ARGUMENTS comment to Counterpart for @Suppress("ACTUAL_FUNCTION_WITH_DEFAULT_ARGUMENTS")

The full compilation error of NO_ACTUAL_CLASS_MEMBER_FOR_EXPECTED_CLASS also makes it clear:

e: file:///home/bobko/jb/test-proj/kotlinx.coroutines/kotlinx-coroutines-core/jvm/test/TestBase.kt:57 Actual class 'TestBase' has no corresponding members for expected class members:

    public expect fun error(message: Any, cause: Throwable?): Nothing

    The following declaration is incompatible because actual function cannot have default argument values, they should be declared in the expected function:
        public actual fun error(message: Any, cause: Throwable? = null): Nothing