The proposal is embedded in this GitHub repository. Easy read, short and clearly written.

From the linked ZDNet explainer:

Apple engineers have put forward a proposal today to standardize the format of the SMS messages containing one-time passcodes (OTP) that users receive during the two-factor authentication (2FA) login process.

And:

The proposal has two goals. The first is to introduce a way that OTP SMS messages can be associated with an URL. This is done by adding the login URL inside the SMS itself.

The second goal is to standardize the format of 2FA/OTP SMS messages, so browsers and other mobile apps can easily detect the incoming SMS, recognize web domain inside the message, and then automatically extract the OTP code and complete the login operation without further user interaction.

Basically, the goal is to automate the process, to have your device enter the code automatically, rather than you having to copy and paste it. Seems to me, in the past when this standardization was raised, there was a security concern about taking the human out of the middle of this process. Was that concern unfounded?