I received a change notification via the Microsoft 365 Admin Center. It says that the Skype for Business Online external access settings are retired.
In other words external access will vanish in the SFBO Admin Center. So, the external access settings can only be managed within the Teams Admin Center or via PowerShell.
In the Teams Admin Center you find the external access settings as follows: Org-wide settings\External access”
Conclusion, opinion and summary
I welcome this change because if you modify external access settings in SFBO or Teams Admin Center it’s reflected, so there’s no need to have two settings pages in different Admin Centers plus PowerShell.