Task breakdown notes:

• copy relevant parts from the proposed OpenAPI spec to ours T307225: 🟧 Add endpoint section for GET /statements/{statement_id} to the OpenAPI specification
• create a service to retrieve item revision metadata (revision id and last modified timestamp) T307227: 🟧️🟪️ Create a service to retrieve item revision metadata
• return json with a single statement
  • T307223: 🟧 Create Use Case and Data Access Service interface for GET /statements/{statement_id}
    • new use case GetItemStatement
    • new data access service ItemStatementRetriever which returns a single statement or null
  • T307238: 🟧 Create GetStatementRouteHandler
    • implement the retriever
    • new GetStatementRouteHandler (create body and headers)
  • don't forget the schema tests T307228: 🟧 Schema tests for GET /statements/{statement_id}
• handle errors
  • 400 in case of invalid statement ID T307239: 🟧 Respond with 400 status code in case of an invalid statement id
  • 404 in case of "statement does not exist" T307230: 🟧 Respond with 404 if the Statement was not found
  • 500 in case of unexpected errors T307226: 🟧 Respond with 500 error response in case of unexpected errors for GET /statements/{statement_id}

Questions (now answered in comments below):

• what should be the error code and error message when the statement ID is invalid?
• what should be the error code and error message when the statement does not exist?
• What do we do when requesting a statement that belonged to an item that was redirected? The statement ID completely changes during a merge (prefix and suffix). It could be useful for the user to know that the item was redirected.

TBD:

• create a task for redirects if we do redirects for individual statements

what should be the error code and error message when the statement ID is invalid?

error code: invalid-statement-id
error message: "Not a valid statement ID: {statement_id}"

what should be the error code and error message when the statement does not exist?

error code: statement-not-found
error message: "Could not find a statement with the ID: {statement_id}"

Finally, regarding

What do we do when requesting a statement that belonged to an item that was redirected? The statement ID completely changes during a merge (prefix and suffix). It could be useful for the user to know that the item was redirected.

It sounds like potentially useful functionality for some use cases. However, there is currently no support in the Wikibase backend/logic that would allow API to know what statement was redirected to. Therefore adding such a functionality would mean a substantial effort that does not seem to be providing enough benefit at this point in Wikibase REST API development cycle.
Therefore, if the input statement ID is an ID of a statement made on the item that was redirected API should treat the statement ID as a non-existent ID.

Ready for verification!

200 - https://wikidata.beta.wmflabs.org/w/rest.php/wikibase/v0/statements/Q11$569c11d8-4564-5981-80e7-189c3841ae3c
400 - https://wikidata.beta.wmflabs.org/w/rest.php/wikibase/v0/statements/X11$569c11d8-4564-5981-80e7-189c3841ae3c
400 - https://wikidata.beta.wmflabs.org/w/rest.php/wikibase/v0/statements/Q11$JOCHEN
404 - https://wikidata.beta.wmflabs.org/w/rest.php/wikibase/v0/statements/Q111$569c11d8-4564-5981-80e7-189c3841ae3c

Looks good, thanks! Verified.