Sidetree is a spec for a collection of Distributed ID methods that uses IPFS to guarantee uniqueness of a DID.
Microsoft started to build Sidetree as the backing for the Ion DID method to improve responsiveness to reads and writes of ID data.
- Sidetree basically intercepts any request for ID reads or writes and tries to cache them to avoid a delay caused by a read or write to the Bitcoin Blockchain.
- As of 2020-11-15 the Sidetree spec did not guarantee that an old key used for signing or encryptions would be available from any of their methods.
- As of 2020-11-15 the IPFS implementations were at Alpha release level and not recommended for production use.
- As of 2020-11-15 the Sidetree spec required two versions of the DID and stipulated that any RP relying on that DID would have to accept either version interchangeably.
- The Sidetree folk introduced the idea of "equivalent" IDs (see above problem) into the DID Core spec just before it went to ba a candidate (2021-04-01) This makes extra work for the RP with no benefit to it.
What is Sidetree?
Decentralized identifiers (DIDs) enable a personor entityto securely authenticate,demonstrate intentand exchange data, wherethe identifier is beingmanagedindependentlyofthose systemsand organizations.DIDevents such as creation, updates, and revocationscan be organizedand propagatedby DID networks.Sidetree describes a protocol for building such networkswithout being coupled to aparticularunderlying ledgeror event recordsystem. The protocol and data model described by Sidetree enablesa decentralized identifiermodel that allowsits usertoultimately be in controlof the identifier and its history,without introducing additional trust requirementsbeyond the underlyingeventrecordsystem.Sidetree-based networks can be implemented on top of a wide variety of eventrecordand propagationmethodologies(public blockchains, federation and witness protocols, or permissioned blockchains)and can be as open as the underlying mechanism. Sidetree enables a scalableprotocol, that can be overlaid on a variety of eventrecord and propagationsystems,for creating vibrant digital ecosystems based ondecentralized identifiers
from Troy Ronda 2021-04-02
- Microsoft introduced did:ion with sidetone and BitCoin support after four years of work. 
- SecureKey Implementations
- https://github.com/trustbloc/sidetree-core-go: Ledger-agnostic Go implementation of Sidetree
- withOrb (https://github.com/trustbloc/orb) and Hyperledger Fabric
- (https://github.com/trustbloc/sidetree-fabric)variations built on top.
- Pamela Dingle, ION – We Have Liftoff!, Microsoft (2021-03-25) https://techcommunity.microsoft.com/t5/identity-standards-blog/ion-we-have-liftoff/ba-p/1441555