FR: enforce @deprecated — the attr ships in all five ports and nothing reads it

[dmealing]

Gap

deprecated and replacedBy are registered documentation common-attrs in all five ports — and nothing reads them. No validation pass, no verify check, no codegen behavior.

A deprecation you cannot enforce is a comment.

Prior art (open source)

dbt pairs deprecation with a date, so a model can be marked as going away on a schedule rather than indefinitely. https://docs.getdbt.com/reference/dbt_project.yml

Direction

Smallest useful version, using vocabulary that already ships:

1. verify warns when a node depends on a deprecated node (via extends, @objectRef, @references, origin @from/@of/@via).
2. Where replacedBy is present, name the replacement in the diagnostic.
3. Optionally, a deprecation date that escalates warn → error once passed.

Interacts with #4 (severity + suppressions): the escalation story is much better if a project can suppress a specific deprecation with a recorded reason while it migrates.

Notes

• Additive and opt-in: a model with no deprecated attrs sees no diagnostics.
• Needs cross-port conformance so a deprecation warns identically everywhere.