Consider adding "Domain Access" (or equivalent) to core

Domain Access and Multisite are not the same thing. The Multisite issue referred to deals with one codebase. Domain Access deals with one codebase&database(one logical set of tables not necessarily in one db).

The University of California uses it for the shared-codebase reason almost entirely. It makes managing multiple websites easier (for the most part, at least in Drupal 7) and also allows us to closely monitor the git repos and the code quality of projects.

By placing modules used by multiple sites in the same /contrib folder accessibly to all websites, we reduce the overall code requirements as well.

Drupal 8 currently has some issues around the auto-loader, which prevents the same module at different versions from operating independently of one another in some cases. If the auto-loader was improved to support this issue would be addressed.

The things mentioned in the ticket (single shared database, shared admin users, modern admin panel) I don't think would be that valuable from a CORE perspective, there are already Contrib modules such as Drupal Remote Dashboard, Acquia Insights (for Acquia platform only), and Aegir that take care of these things should someone want to use them.

I would like to see the multi-site support improved in Drupal 9 in 3 key areas:

1. Improved Auto-loader which can handle multiple instances of the same module in /sites/sitename.com/modules/... folders
2. Improved ability to define in the Composer JSON where things are to be installed, so we can define custom locations of the module in various site folder
3. Improvements to PHP Exception handling overall in Drupal 9 so that potential issues around conflicts of modules as gracefully resolved without the website borking out.

If we are at least on par with Drupal 7 for multi-site I think that would be a good starting ground for discussion.

+1

Hmm, this is sorta confusing as the title states this is about "multisite", while the description is about "domain access". Clarifying.
And maybe this should go to "Core Ideas".

This probably belongs in the Ideas queue so it gets proper attention.

Agree with #6 and #7, moving to Drupal core ideas queue

They are different things. By definition each Drupal site can serve multiple domains. Whether you need Domain Access obviously depends on your requirements. It is not a requirement for serving multiple domains. You can use the Domain Access tools in any site, regardless of being set up as multisite of multiple single sites. Serving multiple domains requires DNS to point to the same web server. For sub domains that is automatic, no need to do anything.

Suggest to transfer this to a feature request for Domain Access. Vote against inclusion with Drupal core. With Composer comes the ability to start a project including any module as requirement. Such a package can be created by anyone. In addition there are already several of those packages, called Distributions. So, don't see a need for it to become part of Drupal Core.

Domain is not yet stable, plan in #3101638: Stable release tasks
The main blockers are config and scattered domain_entity

In the issues where current core workspaces module matured, there was a short discussion that workspaces are an ideal groundwork to re-implement domain module. This issue might be a good place to evaluate that.