How is this a new threat?
If I wanted to scam someone via fear of the IRS I'd take an existing IRS letter and create a new one using that as a template. That's presumably what ChatGPT would do. They don't do that because sending letters costs money, sending emails and leaving automated voicemails costs zero. There are already having to do stuff that sets off alarm bells with any normal person, who knows the IRS is not going to ask for payment in the form of Playstation gift cards or whatever. Having perfect grammar and spelling isn't going to make much difference and arguably that's by design.
The reason there are all the spelling errors etc. in scams is supposedly because the scammers don't want to waste time dealing with people who might have even the slightest clue it is a scam. The bad spelling and grammar leave only the biggest suckers contacting them so they know everyone who does is a real grade A sucker.
Sure, some will claim "it is because they aren't native English speakers" but it seems impossible to me they couldn't go on a board somewhere and post their spiel and ask for English speakers to help clean it up. Even if you had to pay a few bucks for someone's time it would be worth it if it landed ONE more victim! But paying is not necessary, there will always be griefers out there taking delight in helping "the stupids" get scammed - they'll polish that turd to a mirror shine with perfect spelling and grammar! So I subscribe to the idea that leaving the spelling and grammar errors in it is a deliberate choice.