A couple of times when I have accessed demo.moodle.net as admin I have been prompted to upgrade. After clicking through the site works ok and it says it is on the latest 3.4.1.  However, Atto is completely missing; the calendar block is missing and there are other oddities in the upgrade process. See attached screenshots.

Note: Apparently the demomaster site is OK.

I have also noticed that it seems users can delete plugins from the live sandbox site, although I haven't actually tried myself.