Hi
Yes, it should be most changes that cause records to appear. Schedule-Start-Complete was just an example. I think entering values in Attributes (Smarttool, Manual Entry and Part Validation types) is the only change that does not create a record. If you want to see what changes caused records to be created, you can look at all records with the same WorkDefinitionUid and order by Modified and check the columns that changed value.
My concern is that getting billions of records in just 1 year is way more than a normal scenario. Are you really sure that the changes make sense, that this is not a bug? Also I am concerned that even if you completely empty the table today, in 1 year you will have 1TB of new data again.
For the deletion : this is not officially supported, we can't give an official guarantee. But unofficially I can tell you that this table (WorkDefinitionHistory) is not used by CWC, there should be no place that looks at this table, it's only there for debugging, so yes you should be able to remove the records without causing issues. Well, I say without causing issues, but like I mentioned in my previous message please have a database expert to review the situation : there may be performance issues while the delete is happening, and it may use even more disk space temporarily (database logs), and you will probably need to rebuild the index after.