Reclaiming the physical storage of an unused column is often a costly and sometimes impossible operation, which is why many legacy applications end up with the equivalent of my_column_final_final_v2. Database administration requires compromises like this sometimes in the name of uptime and data integrity. Big migrations are always inherently a little risky, and from the view of many DBAs, why even risk it just for a bit of clean up? Your schema shouldn't be totally transparent to your application's business logic anyway, so there are better places to enforce naming hygiene.
I believe in most relational databases you can just alter a column to allow null values and run a series of transactions in the background to set that column value to null, and that will quite effectively free up most of the physical overhead of the column in question. I would be reluctant to delete, rename, or even clear all the data out of a column without providing an alias though.
