The per-account preview switch was kept for the topic overview pages,
which were dropped before they shipped. Its one flag was stored, served
and backed up, and read by nothing; the column-reader audit listed it as
a pending consumer that will not come.
Removed completely:
- `/api/settings/preview-features` (GET, PATCH) is retired: it answers
410 Gone with `meta.errorCode: route.retired` and `removedIn: 1.46.0`,
and the contract publishes it under Retired. The route, its validation
schema, the `PreviewFeatures` OpenAPI schemas and the settings module
are deleted.
- Migration 0485 drops `users.preview_features_json` under a bounded lock
wait, with the retry marker.
- The backup no longer writes the field. A file from v1.44 or v1.45 that
carries it still restores: the schema accepts it and no codec reads it,
so nothing is refused or written for it. The backup-parity goldens
lose the field and apply one setting fewer.
- The data-wipe plan entry and the sharing-surface actor entry go with
the route (frozen count 273 -> 272).
No client build reads the route: neither iOS repository nor any local
checkout references it.
