Simplify feature support matrix - remove network duplication (reference the new registry)

[p-diogo]

Simplify Feature Support Matrix - Eliminate Network Duplication

Problem

Network information, particularly indexing rewards and deprecation status, is duplicated between the Feature Support Matrix and the Networks Registry, creating unnecessary maintenance burden and potential for drift.

Solution

Implement exception-based documentation with clear default policy:

Default Rule: Indexing rewards ⟺ Arbitration support (bidirectional)

Changes

• ✅ Remove "Indexing Rewards" column (now exclusively in Networks Registry)
• ✅ Remove all ~30 standard network rows (exception-based approach)
• ✅ Remove deprecated networks table (now exclusively in Networks Registry)
• ✅ Add governance section per GGP-0062 with clear responsibilities
• ✅ Add graph-node version rationale per Arbitration Charter
• ✅ Add "Default Policy" section explaining indexing rewards ⟺ arbitration
• ✅ Keep empty "Exception Networks" table for future use
• ✅ Keep all core features and data source features unchanged

Benefits

1. Single source of truth: Networks Registry owns all network-level data
2. Less maintenance: Adding networks requires zero feature matrix changes (in standard cases)
3. Clearer for indexers: Simple rule - check registry for network support
4. No duplication: Indexing rewards and deprecations live in one place
5. Future-proof: Exception table ready if needed

Migration Notes

No protocol changes - this is purely documentation restructuring:

• All networks previously listed with rewards still have full arbitration support
• Indexers should check Networks Registry for current network support
• Feature matrix remains authoritative for graph-node features and version requirements

Governance

Per GGP-0062, this change does not require Council approval as it:

• Does not modify arbitration eligibility for existing networks (maintains same policy)
• Is a documentation improvement
• Makes the default policy explicit rather than implicit

Council approval would be required for future changes that add networks to the exceptions table or modify arbitration columns for existing features.

Other notes

• Dependent on https://github.com/graphprotocol/networks-registry (dependency)
• Part 2 of 2 for eliminating indexing rewards duplication