Main issue with adding something you might need in the future is that people leave, people forget, and then, one year later, there's some metadata column but no one remembers whether it was already used for something. Can we use it? Should we delete it? Someone remembers Knight Capital and spectacular failure when old field was reused. So it's always safer to keep existing field and then you end up with metadata and metadata_1. Next year no one remembers why there are two metadata fields and is very confused on which one should be used.
Easy solution: Add a comment in your schema-creation SQL script explaining what the purpose of the column is. Or some other equivalent documentation. Stuff like that should be documented in any case.
So, every db-column gets a "in_use_boolean" assigned? It gets reset every year, reset on first use query and auto-purged after a year and a day. Self-pruning database..
This would break if you need something after two or three years. It happens.
My point is - it's relatively easy to tell if something is used. Usually, a quick search will find a place where it's referenced. It's much harder to 100% confirm that some field is not used at all. You'll have to search through everything, column names can be constructed by concatenating strings, even if it's strongly typed language there could be reflection, there could be scripts manually run in special cases... It's a hard problem. So everyone leaves "unknown" fields.
