UPDATE
In April 2021,
CryptoTaxAudit.com published the information requested by the IRS in crypto audits. Some spellings are different (e.g. "I.D." vs. "ID"), one typo was corrected ("in paper" -> "on paper"), but other imperfect wordings remained. The letter contains three additional items vs. the /r/bitcoin letter:
7. All records reconciling acquisition, whether by purchase or cryptocurrency-to-cryptocurrency (C2C) transaction, and sales, or other disposition such as C2C transaction histories or receipts and counterparties to all transactions
8. All records relating to investment expenses claimed with respect to any investments in virtual currency, including but not limited to invoices, receipts, canceled checks, bank records
9. All records relating to any lending or borrowing (on margin or otherwise) through fiat currency or in cryptocurrency