When will be support for Nikon Z8?

This support page details the RAW formats natively supported by iPadOS17:

Digital camera RAW formats supported by iOS 17, iPadOS 17, and macOS Sonoma - Apple Support

You may find that some Apps support a wider range of RAW formats than provided natively by iPadOS. Perhaps consider Stratopherix FileBrowser - Document Manager, or its “pro version” FileBrowser Professional:

https://apps.apple.com/app/filebrowser-document-manager/id364738545

https://apps.apple.com/app/filebrowser-professional/id854618029

You might review the extensive online Manual and help files to determine suitability for your needs.

https://www.stratospherix.com/products/filebrowserprofessional/filebrowser-professional-user-guide.php

Apple does invite submission of comments and feature requests via its Product Feedback portal. If you would like to see native support for additional RAW formats in a future version of iPadOS, this would be the most appropriate channel trough which to make a request:

Feedback - iPad - Apple

I phoned Apple Support today. The senior advisor that I spoke to asked me to pass on to others who use the Z8 to please provide feedback under Apple Support feedback feature to request that they expedite the compatibility of the Nikon Z8 with Apple photos. The more people they hear from, the sooner it will happen.

Sorry, but only Apple knows about upcoming features.

However, you can let Apple know about your wishes by using the Feedback link:

Product Feedback - Apple

Maybe also tell Nikon Support about that.