The Data module has long been replaced with content apps in Magnolia 5 (see https://documentation.magnolia-cms.com/display/DOCS/Data+module). Even though it is "hidden" and not directly accessible through M5 UI, it has come the time to remove it for good. There also are documentation and Task(s) to migrate from data to content apps https://documentation.magnolia-cms.com/display/DOCS/10.+Migrate+from+4.5+to+5#id-10.Migratefrom4.5to5-Replacecustomdatatypeswithcontentapps