The User - Profile History plugin is an incredible tool, useful outside of the GDPR scope. On one site I manage, there are nearly a dozen admins. With that many fingers in the pie, this plugin has come to the rescue many times. It stores the previous value, the new value, and the user who made the change. Finger pointing doesn't happen anymore, and admins are actually more careful about what they do, knowing that their actions are being recorded.
Let's get back to GDPR implementation! Where this plugin fits into the GDPR puzzle is the Right to Access provisions. Specifically, when their data is being processed as it relates to the Joomla site and maintenance of their user account.
Out of the box, this plugin monitors standard Joomla user fields (name, username, password, email), assigned user groups, basic settings (timezone, editor, language, etc), the Joomla User Profile plugin (and others that are implemented in that manner), and Joomla Custom User Fields (such as the TOS). It is written in such a way as to allow additional plugins to monitor other areas and components within Joomla. If you need something custom monitored for changes, please contact me to discuss your needs.