That list is just those chose to publicize their grid. For an exact count there have been 3,748 of them since April 2020, which is over 90% of the grids EVER in use, anywhere.
DreamGrids suitcase may be set to on or off, which is a grid owners choice. Just like OsGrid, which has it off. It takes a lot of effort to extract UUIDS from visitor inventories. The people who have these skills are usually running linux. But it is just a click to save an entire region to disk, so likely no one bothers to do what you fear. I seriously doubt anyone has detected things they made suddenly leaking out of inventory.
Since DreamGrid is essentially stock Opensim, they are no more or less secure than most grids running Opensim. The serious security issue is not in Home computers or Windows, but the severe issues in some versions of Opensim, especially on Linux, where anyone can read the hard drive or your LAN, such as /etc/password or /etc/shadow. This is still on the loose everywhere in many of the Linux grids. The last fix from DreamGrid ( thanks, Marianna, who reported it) is to stop people from placing Russian flags or obscene images on your map.