RE: Will Customization pages and Administration pages of D365 be switched to Unified Interface before October 1, 2020?
Might not be what you are asking, but if you do not find something in the modern options Henry suggested - you can still get to the traditional Settings area, containing same Customization and Administration. Example of this is Data Imports that normal users might need to get a hold of, this is still possible and does not require delaying migration to Unified Interface.
On top right corner of the menu click on the settings gear icon which opens a drop-down then click "Advanced Settings"
This will bring you into focused web client app contained to just Settings area, this works even if you enable Unfied Interface only mode.
There you will find completely all settings and administration capability you are used to, depending on your specific user permission level.
