We know this to be true as Phil said a number of times he received things off his wishlist and even checked his phone to be sure. You could argue other things on the list aswell, but I limited it to unreported income. Now we cannot be sure he does not report this as income, but I doubt he does. If I wanted to theory craft I doubt he reports big donations like the 1000 USD one and Portal Gun sales either. False Deductions would also be a thing to look at.
In terms of his current taxes, he's in the clear on the wish list stuff since he hasn't had to file anything since making it. Big donations I'm almost positive he reports since they go through PayPal. PayPal automatically generates a tax earnings form for you to file. I believe Twitch/Amazon does the same as well so anything from subs/cheers should be reported as well. Would he have to report the portal gun sale? That seemed more like a private sale as opposed to something he did through his business.
Not emailing them is the right move. I think an actual tax saga would be very interesting, and I'd be lying if I said I wouldn't do a deep dive on any kind of financial records DSP released just because his spending has always been such a mystery. If nothing else it'd answer the question of whether he's just been hoarding his money all this time or actively spending it on stupid things/repaying debts etc. But, like with all things Phil, he'll be his own worst enemy. If he is genuinely trying to cheat the system, he'll get caught because of a stupid mistake of his own doing, not from anything us or any other group does.