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.
