An attached statement has always been required in order to make the election.

If I had to guess, that line was added to the 1040 because (a) people were treating it that way without including the required information so this is a reminder that they can't randomly do it without an election and/or (b) to help the IRS computers look for the election.

Here is the direct link to the section in Publication 519 that lists the information that is required to be in the statement/election:

https://www.irs.gov/publications/p519#en_US_2023_publink1000222196