If your script is named "-i", and in your path, just enter "-i", and /bin/sh -i is executed which gives you an interactive shell.

There are probably half a dozen other ways to trick the shell into executing arbitrary code that is not contained in the script (more if the script actually does anything non-trivial, like e.g. an installer of some sort). So instead of trying to fix them all (and most likely missing a few), everybody just agreed that it was a terrible idea and removed the feature.