[Feature]: Remove deprecated ai-plugin.json recommendation

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Problem Statement

AX Score currently recommends creating an ai-plugin.json manifest and hosting it at /.well-known/ai-plugin.json. However, ChatGPT Plugins were fully deprecated and shut down in April 2024.

Timeline:

• March 19, 2024: Users could no longer start new plugin conversations
• April 9, 2024: All plugin-based chats were permanently disabled
• Replacement: ChatGPT Plugins were replaced by Custom GPTs with Actions, which use a completely different mechanism

The ai-plugin.json format is officially dead. No AI system currently reads or utilizes this file. Recommending it to users is misleading and wastes their development time on implementing a defunct standard.

References:

• OpenAI Community: Have plugins been replaced completely?
• ChatGPT Retrieval Plugin - Deprecated Docs (GitHub)
• Your Everyday AI: ChatGPT is Killing Off Plugins

Proposed Solution

1. Remove ai-plugin.json from the scoring criteria and recommendations entirely
2. Replace it with more relevant AI discoverability checks:
   • llms.txt detection (emerging standard, documented at llmstxt.org)
   • Enhanced JSON-LD structured data analysis (WebSite, Article, Person, BreadcrumbList schemas)
   • RSS/Atom feed detection
   • AI crawler allowance in robots.txt (GPTBot, ClaudeBot, PerplexityBot, etc.)

Alternatives Considered

• Keep but lower weight: Not recommended — the spec is literally dead with no consumers.
• Replace with MCP (Model Context Protocol): Could be relevant for API services but too niche for general scoring.

Use Case

As a website owner running npx ax-score, I want the recommendations to reflect currently active standards, not deprecated ones. Implementing ai-plugin.json today provides zero benefit to AI discoverability.

Additional Context

This is arguably a bug rather than a feature request, since the tool is recommending a discontinued standard. Prioritizing this fix would improve the tool's credibility.