My solution to this is to rename that table/column to “table_deprecated” or “column_deprecated”. This has the nice property of being reversible, causes nice visible errors if something unanticipated is actually using the column/table, makes it obvious that it shouldn’t be used (well, one would hope), and makes it easy to find for permanent deletion later.

That's a great idea! Although I did work with a person once who would use anything that looked helpful regardless of "deprecated" being all over the place (Eclipse would even yell at the method calls but he didn't care). Granted that's Java and at that point in time APIs were being deprecated without workable replacements, so it's hard to fault him.

At that point you might as well just delete them.

I think you're missing the point. If your migration renames a table or column, then the migration can be rolled back, by simply reversing the rename. If you delete the thing, then there is no way of rolling back other than restoring from backup.

I can see it being useful to rename these things, as long as there is some process in place to delete the renamed items after a period of time has passed.

Would be good if you could schedule a future migration that will happen only after a certain date has passed, which deletes the renamed tables/columns.

This sounds so much like something that I'd do, that I had to stop and think about my personal hall-of-fame screwups to make sure that wasn't one of them.

In terms of things that scare me as a developer, stale data is WAAAAYYY less concerning then having people try to shotgun fix an issue on a production server after an irreversible update took down the entire system... Especially now that the developer and the entire organization is likely now in a panic state and not thinking clearly.

That bug you describe, maybe stale data is part of the problem, but ... why are you having a new person working on the billing system presumably without any code review? It sounds like you have a lot of process issues.

Just create an issue in your issue tracker with the highest priority to delete a column (and why) with a deadline in 4 weeks. Stupid simple solution but works perfectly.