Hello, up to 10.4 using mosaics for server required Image Extension.
For later versions it does not appear in the list of extensions for ArcGIS Server in ESRI docs.
Is it still required for publishing a mosaic?
Yes - Image server is now considered a server 'role' rather than an extension.
ArcGIS Server licensing roles—Documentation | ArcGIS Enterprise
Key concepts for image services—Documentation | ArcGIS Enterprise