If Gutenberg is not installed or activated on a site, the callout will be displayed to users with the install_plugins capability. If Gutenberg is activated, the callout will be shown to users with the edit_posts capability.
Limiting the visibility of the callout to a specific subset of users will avoid inundating the WordPress.org support forums with Gutenberg support requests. A concern that was expressed by Daniel Bachhuber four months ago.
“There’s a good amount of fit and finish to be desired before rolling out to a large number of unacquainted users,” Bachhuber said. “We’re not quite over the hump on the issues we do already know about. There’s not a ton of sense in having 10,000 users re-report them.”
Other than this change, the change log from WordPress 4.9.8 RC 1 remains the same. WordPress 4.9.8 was originally scheduled to be released today, but has been rescheduled for Thursday, August 2nd.