Google has recently added new information around Vary: Cookie support in the signed exchanges Google Search help documents. This comes shortly after Google decided to bring support signed exchanges for some desktop sites.
You can find the new details on the Signed Exchanges help document under the implementation section, where it says this new line:
To make sure that content displays properly when served on multiple devices, do the following. Move personalized content, such as shopping carts, into lazy-loaded elements that are outside of the SXG. Alternatively, add the Vary: Cookie signed header; SXGs with this header will be shown only to visitors without a cookie for your site.
The line use to say "Move personalized content, such as shopping carts, into lazy-loaded elements that are outside of the SXG. For instance, only sign resources where the Cache-Control header includes the public directive."
Now the line says "Move personalized content, such as shopping carts, into lazy-loaded elements that are outside of the SXG. Alternatively, add the Vary: Cookie signed header; SXGs with this header will be shown only to visitors without a cookie for your site."
That seems to be the full change here.
Forum discussion at Twitter.
