Form 8453 is the cover sheet for documents you are required to send to the IRS. Usually the "acceptable continuation sheet" for Schedule D is a copy of your brokerage statement or something similar, such as Form 8949.
If you only entered the summary figures from your 1099-B, then the IRS wants to see your full brokerage statement.
If you entered cryptocurrency and they sent you completed Forms 8949, then the IRS would like to see those.
