The deprecation of the multisite feature as been discussed in #2306013: Multisite is a valued feature that will not be deprecated. → .
The problem of the Drupal multisite feature is not the feature in itself but what it does not offer natively compared to other CMS: single shared database, shared admin users, modern admin panel...
Even with Domain Access → module which is useful, it's not really made to manage a lot of websites (for example selection of a domain is done via simple combobox/checkbox which does not allow any filtering, grouping...) and there is still lack of mentioned features.
Look at what typo3 or eZ offer natively, it's valuable to compare with competitors. We have some catching up to do... Natively or via module.
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The Ideas project is being deprecated. As discussed in a core committer meeting issues that are adding modules are being moved to the Drupal CMS project for discussion.
This isn't something we would add to vanilla Drupal CMS but I think it would be great as part of a site template.