I don't think it is "unforgivable". I reckon it might be unforgivable if Labor had preferred One Nation over the Greens; but to just not give any preference at all is... not really that bad in my opinion.

On the one hand, I think voters should never use how-to-vote cards at all. Voters should build their own understanding of which parties align with their values, and vote accordingly. And so Labor not suggesting any preferences seem fair enough to me.

But on the other hand, a party's listed preference is a strong indicator of how that party sees itself in the political landscape. Its an indicate of how their values align with the values of others, and hence an indicate of the parties values and priorities. So although I don't think voters should follow the how-to-vote cards, they can be useful sources of information. And in that sense, Labor saying "we have no preference regarding the other parties" is effectively a way of saying they agree with One Nation just as much as they agree with the Greens. And that could be seen as unforgivable if interpreted in that way.