typespec1-11  Exercise 1: A Compatible Change to a Published Version
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The change (branch ex1-subtitle, in the scratch repository), added to the Book model:

  /** A subtitle, when the book has one. */
  @added(Versions.v2) subtitle?: string;

Regenerated and committed. Summary of the pull request:

   api/main.tsp                 | 3 +++
   api/openapi/openapi.2.0.yaml | 6 ++++++
   api/openapi/openapi.3.0.yaml | 6 ++++++
   3 files changed, 15 insertions(+)

ci.sh:

  PASS  tsp format --check
  PASS  tsp compile --warn-as-error
  PASS  generated OpenAPI is committed
  PASS  no breaking change vs main
  ALL CHECKS PASSED

oasdiff changelog, main's openapi.2.0.yaml against the branch's (first 12 lines):

  4 changes: 0 error, 0 warning, 4 info
  info	[response-optional-property-added] at api/openapi/openapi.2.0.yaml
  	in API GET /books
  		added the optional property `items/subtitle` to the response with the `200` status

  info	[new-optional-request-property] at api/openapi/openapi.2.0.yaml
  	in API POST /books
  		added the new optional request property `subtitle`

  info	[response-optional-property-added] at api/openapi/openapi.2.0.yaml
  	in API POST /books
  		added the optional property `subtitle` to the response with the `200` status

IT PASSES. oasdiff reported 4 changes, all 'info': an optional property added to two responses (GET /books and POST /books) and a new optional request property on POST /books.
NOTE what else changed: openapi.3.0.yaml also gained six lines, because version 3 contains everything version 2 does, so the new property appears in both published files.

WOULD I ACCEPT IT?
  By the script's definition of 'breaking', yes. By the habit that a published version is a promise, I would ask for it to go into the next version instead. The reasons it is compatible are real (optional, and a client that ignores unknown properties is unaffected), but a client that validates responses strictly against the version 2 file, or a generated client with a closed model, could see a difference; I did not test either. The script cannot decide this: it is a policy choice, which is why Exercise 2 exists.

WHY THIS WORKS AS AN ANSWER
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It records a case the check passes and a human might still question, and says what I did and did not check.