D6: Deprecate Legacy Environments

# D6: Deprecate Legacy Environments

## Type
Task / Epic

## Priority
Medium

## Description
As part of the D6 modernization initiative, legacy environments need to be formally deprecated and removed from active support. This includes identifying outdated environments, communicating timelines, migrating remaining dependencies, and disabling access after the deprecation window.

The goal is to reduce maintenance overhead, improve platform consistency, and ensure all teams are operating on supported environments.

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## Scope
- Identify all legacy environments currently in use
- Audit services, integrations, and dependencies tied to legacy environments
- Define migration path to supported environments
- Communicate deprecation timeline to stakeholders
- Disable creation/use of deprecated environments
- Remove infrastructure and documentation related to legacy environments

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## Acceptance Criteria
- [ ] Inventory of legacy environments completed
- [ ] Migration plan documented and approved
- [ ] Stakeholders notified of deprecation timeline
- [ ] Remaining active workloads migrated or formally exempted
- [ ] Legacy environments disabled/decommissioned
- [ ] Documentation updated to reflect supported environments only

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## Notes
- Coordinate with platform/infrastructure teams before shutdown
- Ensure rollback or temporary access process exists during migration window
- Track dependent teams and unresolved blockers separately