The message informs that the Legacy SharePoint Invitation Manager settings have been retired, with the specific setting for guests to sign in with the invited account no longer appearing. The SharePoint Invitation Manager's retirement doesn't change product behavior. It recommends using the 'Anyone' link sharing setting and provides a link for configuring external sharing. No admin action is required before the rollout.
Applies to: SharePoint Online · OneDrive for Business
Source: mc.merill.net — data via Merill Fernando's open archive (MIT).