The homework justification didn't replace anything else that was there before. But we supplement it with (up to two) new custom off topic reasons (just as we created the homework one).
When there is no consensus among the people voting to close a post, the site displays the default off topic notice:
This question does not appear to be about economics, within the scope defined in the help center.
so one possibility would be to introduce this as an explicit off-topic option for people to choose. The danger I see in that strategy is that almost any reason for closing would match this statement. It would probably be best if the off topic reasons were (at least roughly) mutually exclusive.
Alternative suggestions are welcome.
I have added a new custom off-topic reason matching the quoted text above. But it requires a second moderator's input/approval at here.
For the record, we are able to add one more custom off-topic reason if necessary/desired (the site maximum is three).