Code change:
The user agent includes stored cookies in the Cookie HTTP request header field. <del> When the user agent generates an HTTP request, the user agent MUST NOT attach more than one Cookie header field. </del> A user agent MAY omit the Cookie header field in its entirety.  For example, the user agent might wish to block sending cookies during "third-party" requests from setting cookies (see third-party- cookies). If the user agent does attach a Cookie header field to an HTTP <del> request, the user agent MUST compute the cookie-string following the algorithm defined in retrieval-algorithm, where the retrieval's URI is the request-uri, the retrieval's same-site status is computed for the HTTP request as defined in same-site-requests, and the retrieval's type is "HTTP". </del> <ins> request, the user agent MUST generate a single cookie-string and the user agent MUST compute the cookie-string following the algorithm defined in retrieval-algorithm, where the retrieval's URI is the request-uri, the retrieval's same-site status is computed for the HTTP request as defined in same-site-requests, and the retrieval's type is "HTTP". </ins> 5.7.2.

Related discussion:
text: If we standardize different behavior for default vs explicit SameSite=lax … We increase the complexity of SameSite cookies significantly. Is that desirable? Will developers be able to know the difference without this effectively being displayed as an additional cookie attribute in developer tools? Are we arriving at the desired end state or piling on more legacy and technical debt? It looks to me like this should instead be a fourth SameSite value, beside none, lax, and strict. Then we’ll have to decide if the new, fourth value is the default or the old explicit lax. I generally don’t like fixing legacy problems by increased spec complexity since it doesn’t help the web/Internet progress. I will bring this up with the CFNetwork folks at Apple to see what they have to say.
Hi John, You bring up some good points. I'm curious if the CFNetwork folks had anything to add.
I think what's under discussion is making the fourth value explicit in the specification. It apparently is already a thing in some implementations. I'm not sure we have a good reason to expose it in syntax though. But yeah, a clear name in the specification would help with tooling and such.
+1
"Default" is an explicit value for the in the algorithms and storage model defined in Section 5. It's not a part of the syntax. Unfortunately it looks like the algorithm only sets the if there's an explicit SameSite attribute in step 17 ("Default" is used for an otherwise unknown value). Presumably the should be initialized to "Default" somewhere, but that's an assumption and not explicit. The value of same-site-flag is technically undefined if there isn't a SameSite attribute and that's clearly not the intent.
Hi, Zhenchao Li from Apple working on CFNetwork. This change sounds quite complex to me. Although, if it means minimizing website breakage and tightening cookie restriction, I'm in favor of explicitly adding the behavior to the RFC. It does sound like we need to move from "SameSite=lax by default" to "SameSite=[fourth value] by default". Also, "default" still needs to be a separate state persisted in "Storage Model" so as not to be conflated any time.