Just to be clear: since last year (2020), TurboTax' online version has two distinct points in the flow where you can upload your 1099-B:
- At the start: This upload is to pre-populate the values for you
- At the end: This upload is to provide the IRS with a copy of the individual transaction details
Folks here are mainly talking about the second point (at the end). If you uploaded your 1099-B then, then TurboTax claims that they will send the copy to the IRS for you.
It's hard to be certain, but I think it's most likely a bug that the filing instructions still ask you to mail in Form 8453 separately. Since TurboTax now specifically includes a new step to upload the 1099-B for the express purpose of sending a copy to the IRS, I think it's reasonable to assume that that's what they will do.
So I for one will not bother mailing in a physical copy of Form 8453 and the documents.
