Officially Customizations through extensions only in dynamics 365?

In the previous releases (before Dynamics 365 Spring ‘18 release)

Customize with extensions and overlayering allowed 

you can check this link for more details 

https://docs.microsoft.com/en-us/dynamics365/unified-operations/dev-itpro/extensibility/customization-overlayering-extensions

In Dynamics 365 Spring ‘18 release , Customizations through extensions only

Source 
https://community.dynamics.com/365/financeandoperations/f/765/t/276371

just download the release notes

and check Customizations through extensions only page 65

you can see

Thanks