Comment by dsissitka

If you're a Linux user you might like Firejail for this.

  firejail --appimage --net=none --private=~/path/to/jail ~/path/to/Obsidian.AppImage

--private=~/path/to/jail limits access to your home directory to ~/path/to/jail and when you don't want Obsidian to have internet access you can take it away with --net=none.