Dear MMoore59,
Good day! Thank you for posting in our community. Your question is "New Sharepoint Header functionality - was there, now has gone".
Per testing, it is also not appearing at our end. Then I checked the M365 roadmap and found this feature is a new change and didn't finish rolling out yet. I am afraid you will need to wait for longer time until it is fully released.
If you want to get a try to the new released features earlier, you can set your tenant to "target release". When new features start rolling out, it will be roll out to "target release" tenants first before standard tenants.
Set up the Standard or Targeted release options - Microsoft 365 admin | Microsoft Learn

Thank you for your cooperation and have a nice day!
Best Regards,