Configuration opt-in safety

When introducing new configuration options or making changes that could be disruptive, make them opt-in by default rather than enabled automatically. This prevents breaking existing functionality and gives users control over when to adopt new behaviors.

Key principles:

• New features should be disabled by default with explicit flags to enable them
• Potentially disruptive migrations should require explicit opt-in
• Breaking configuration changes should be avoided, or made opt-in with clear migration paths
• Validate configuration options and provide clear error messages for unsupported settings

Example from migration configuration:

// Bad: Automatically migrating potentially disruptive changes
export class NgClassMigration {
  // Always migrates multi-class keys, potentially creating verbose output
}

// Good: Making disruptive changes opt-in
export class NgClassMigration {
  constructor(private options: { migrateMultiClassKeys?: boolean } = {}) {}
  
  migrate() {
    // By default don't migrate multi-class keys
    if (!this.options.migrateMultiClassKeys) {
      return; // Skip potentially disruptive migration
    }
    // Only migrate when explicitly requested
  }
}

This approach protects users from unexpected changes while still providing access to new functionality when they’re ready to adopt it.