for non-dark pattern use cases where it is beneficial from a usability standpoint to share cookies across domains
Yes, I think there are some cases where a user might want to opt-in and allow cookies to be shared across a specific set of domains. The main reason I could think of it to allow cross-site business integration to continue to work as (currently) designed.
Well, what are those use-cases? That is what I asked.
SSO across multi-domain, Github logins, using Youtube.com and Gmail.com with the same G account. Assuming this is done the same way as existing Multi-containers feature