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.
This article explains that if you delete a field that's unique to 1 content type then it's deleted from the database. However if this field is being shared by at least 1 more content type then the field database table remains however the data of field is removed (the data corresponding to the content type that the field is being removed from).
If the field you delete is unique to a content type, then the field is gone from the database (that includes it's data). If you delete a shared field from, the field is removed from that content type along with the data unique to the contenty type.