What is an effective way to wipe sensitive data on a data pages after dropping a table's column?
No, sorry, there is no feature for reinitializing empty pages or extents when they are unused.  

It is true that if you take a backup and create a new database from that backup the empty extents are not backed up, so that data could not be restored to the new database. But that still leaves some old data in the extents that are still in use. See: 

&#39;Cleanse&#39; a SQL Server database file created with Instant File Initialization enabled?

I suppose that overwriting the data first, then deleting it, might help.  But that may not be practical for many reasons. (Too much data to update, constraints, unique indexes, triggers, etc.)