Deprecate and remove legacy settings screen (pre-3.2.0)

[jeffpaul]

Is your enhancement related to a problem? Please describe.

In December 2024, version 3.2.0 introduced a refactored settings screen. The legacy settings experience was intentionally left available via a filter for backward compatibility, but it is no longer supported. All service providers and features added since 3.2.0 only exist in the new settings experience, making the legacy screen effectively dead code.

We should formally deprecate and fully remove the legacy settings screen and its associated filter in a staged, explicit way.

Designs

Proposed approach:

1. Deprecation notice release (major release)

• Show an admin notice when the legacy settings filter is in use
• Add a clear deprecation notice in the Upgrade Notice section of readme.txt
• Add a changelog entry explaining that the legacy settings screen is deprecated and scheduled for removal

2. Filter removal release (major or minor release, depending on timing)

• Remove the filter that allows the legacy settings screen to be displayed
• Continue to leave the underlying code in place temporarily to avoid hard breaks

3. Full removal release (major release)

• Remove all legacy settings screen code and related logic
• Clean up any remaining references or conditionals tied to the old experience including within the docs site
• Include a final changelog item noting the hard deprecation

Acceptance criteria:

• Admin notice only appears when the legacy settings filter is active
• Deprecation is clearly documented in readme.txt and the changelog
• Legacy settings filter is fully removed before the underlying code is deleted
• No new features or providers support the legacy settings experience during this period

Note:

This is intentionally a multi-release process to give developers and site owners clear notice and time to adjust, while reducing long-term maintenance and dead code in the plugin.

Describe alternatives you've considered

No response

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