Simple API format transition to PEP 691 JSON on /simple/ path

The Simple API format is changing under Warehouse from legacy HTML to a PEP 691 JSON representation while keeping the same /simple/ path. The affected feature is package index access, where clients may receive JSON instead of HTML and pagination shifts from page query parameters to Link header based pagination.

Authentication for uploads has also moved toward scoped API tokens and Trusted Publishing, with legacy password uploads retained for backward compatibility. The compatibility boundary is relevant for index clients and upload tooling that assume HTML parsing or password based authentication.

Documentation describes deprecation intent for the legacy HTML Simple index and password uploads, but a firm sunset date and final removal criteria are not published. This leaves planning for long term client support uncertain and version sensitive to Warehouse configuration.

What criteria will be used to decide removal of the legacy HTML Simple index? Is there a published timeline or decision process for sunsetting password uploads? How should clients reliably detect which Simple API representation is being served for a given request?