Cancelled Business/Company

It is illegal, however the ABR site is often out of date. They take a while to list things.

I'm not sure if an ABN once cancelled can be reinstated. If the action is causing you concern a call to the ATO should sort it out.

As a company they need to still be activated but with a business (sole trader) there are several ways that a business may come under a hobby business and not require a ABN.
The ABN look up site has taken 6 months before my ABN was reactivated after being listed as cancelled although it never was, just wrong on the site. If it is your own business ATO is helpful but if someone else's I am not sure what you can do.