Though, just to be clear, the per-user ones are also public. They're just a convention where if you make a subpage of your user page and call it "Sandbox", nobody is going to complain about the encyclopedic value of your edits.

If you really want something private, there is https://en.wikipedia.org/wiki/Special:ExpandTemplates (or of course just hit preview and dont save)

True , though I just discovered category scans still hit your user sandbox. Kind of silly