Hibernation requires an EC2 instance to be an encrypted Amazon EBS-backed instance. This ensures protection of sensitive contents in memory (RAM) as they get copied to EBS upon hibernation. You can now enable Amazon EBS Encryption by Default, to ensure all new EBS volumes created in your account are encrypted. You no longer have to create an encrypted copy of the Amazon Machine Image (AMI) before launching an instance. Alternatively, you can also now specify encryption intent at launch (with unencrypted AMI or snapshot) and enable hibernation at the same time (see link).
Hibernation is available for On-Demand and Reserved Instances running on newly launched M3, M4, M5, C3, C4, C5, R3, R4, and R5 instances running Amazon Linux (1). With this expansion, EC2 Hibernation feature is now available in the US East (N. Virginia, Ohio), US West (N. California, Oregon), Canada (Central), South America (Sao Paulo), Asia Pacific (Hong Kong, Mumbai, Seoul, Singapore, Sydney, Tokyo), and Europe (Frankfurt, London, Ireland, Paris, Stockholm) AWS Regions.