Do Decentralized Identifiers Require a Blockchain?

Blockchain is great but is it necessary to use it when implementing decentralized identity?

Further Reading:

• Decentralized Identifiers (DIDs) Explained
• Overview of Decentralized Identities
• Verifiable Credentials Explained
• Decentralized Identifiers and Verifiable Credentials: The Building Blocks for Self-Controlled Identities