As usual you can find such information in the doc/bc folder in eZ Publish root folder. That folder was introduced in the 4.1 version to track all BC changes and changed behavior. We have published also changes in the alpha, beta, RC release announcements so if you followed them you should have overview what was changed.
form module was very old module used for sending contact messages. Ref. to old doc: http://ez.no/ezpublish/documentation/reference/modules/form It was insecure and poor implemented. Form replacement is information collection mechanism in the current eZ Publish versions.