Progress of migrating C4/M4 instances to nitro hypervisor.

In this article: https://perspectives.mvdirona.com/2021/11/xen-on-nitro-aws-nitro-for-legacy-instances/ it mentioned that nitro can support legacy instances. But I can't find an official notice of this. Can you confirm this article is correct and if so, can you tell if M4/C4 instances are now supported or a time frame. Also, when they are supported, does this mean that the bidirectional serial console will also be supported for this legacy instances?