What happens when you delete a field from an existing Content Type? When you delete a field from an existing Content Type, does the mysql field also get deleted or is it still there except it's simply not used? This is whether or not the content type already has data.
When you delete any of the field of the content type then all the log of that field from the database is deleted.

eg. Suppose we create a field named test in the content type A then it creates 2 tables in the database ie field_data_test and field_revision_test.

So when you delete the field test then both the tables are also deleted from the database.