deprecate

MasterMind: Deprecate

Every other skill adds or fixes. This one deletes, migrates and retires: the half of engineering where the reward (a smaller system) arrives long after the risk, and where what breaks is whatever you couldn't see was still reading it.

The shape: expand → migrate → contract

Any removal that can't land in one atomic commit takes three phases, in this order, each shipped on its own:

• Expand: add the new thing beside the old. Both work. Nothing has moved yet, and stopping here breaks nothing.
• Migrate: move consumers in batches sized by blast radius: one caller at a time where a mistake pages someone, a whole service where it doesn't. Verify each batch before starting the next.
• Contract: remove the old, once nothing reads it.

Renaming users.email_addr to users.email on a live system, as five deploys:

1  add `email` · backfill · write both · still read `email_addr`   ← expand, fully reversible
2  read `email` · keep writing both                                ← the only behavior change; watch it
3  move every caller off `email_addr`, batch by service            ← migrate
4  stop writing `email_addr` · leave the column for one rollback window
5  drop the column                                                 ← contract

Each line deploys and reverts on its own; the sequence is the safety. Collapse any two and you get the classic outage: during a rolling deploy both versions run at once, so the instance that hasn't restarted yet is still writing to the column you just dropped.

Prove it's unused: never assert it

Never contract on a search you didn't run, and never treat one symbol search as the answer. "Nothing references it" is a claim about the whole system, and the places that break are the ones a symbol search can't see: config and environment, infra and CI, docs, runbooks and dashboards, other repos and clients you don't build, and every dynamic reference: a name assembled from a string, reflection, a container registration, a route table, a serialized payload, a stored default, an analytics event name. Search for the spelling, everywhere it could be written, not the identifier inside one tree.