Mine is taking up ¾ of the hard disk space. The "Free Up Space on your Mac" addresses the obvious places to get thin out files, but the System Data file appears to be integral with the MacOS that isn't addressed by those options. I see the spinning beach ball of death way more than I should, almost every action in Lightroom or On1 Photos Raw is held up. I addressed this with Apple support, even up into the upper levels, without any improvement. Did everything they suggested and still with ¾ of the disk space taken up with System Data.
Somehow need to get Apple involved, not just among the community to address this issue. Obviously, an OS issue that they need to address, that has been a problem for over a year. The problem is how to get them to do it. I don't even know if they ever even look at the issues brought up here in the Apple Community. Very frustrating.