The ability to enable public sites during or after SharePoint Online site creation will still be accessible. This change will not affect the settings of existing sites.
This update will gradually rollout as follows:
We will start rollout across Outlook in the coming months, starting with Outlook on the web.
The setting change for SharePoint Online Group-connected team sites will roll out to Targeted Release organizations at the end of April, and will roll out to all Office 365 organizations over the following months.
This update was announced on Message center on April 19, 2018 via the following MC134487.