You are correct, at Etsy we deploy schema changes once per week. The code always works against both versions of the schema. We never take downtime for schema changes.
We avoid data loss by doing soft deletes as much as we can. Sometimes we do want to do real deletes, especially for data that is heavily denormalized, but in those cases we keep an audit trail so that the data can be reconstituted in the event of a mistake.
We avoid data loss by doing soft deletes as much as we can. Sometimes we do want to do real deletes, especially for data that is heavily denormalized, but in those cases we keep an audit trail so that the data can be reconstituted in the event of a mistake.