Will Apple Bring Back Print Services???

I really do not think the Apple Print Services will come back, but what you can do:

Create a PDF of your book project (at least this works with High Sierra) and send it - for instance in Germany -

to photographerbook.de. The even offer a decent description of how to do it: https://www.photographerbook.de/fotobuch/fotobuch-foto-app-iphoto/

I had a 100 page 28x21 cm hardcover book printed a week ago and found it as good as the former Apple books (and it is less expensive).

So, if you still have a High Sierra system, I advise you to keep it on an additional hard disk before you upgrade the main disk to Mojave (one never knows what is going to happen with Photos in the near future, and hard disks are really cheap nowadays).

I am an Aperture fan and so I can also use Aperture (under High Sierra) even if there are problems under Mojave.

We are not allowed to speculate in these forums. The only written statement by Apple I have seen is this:

Use Project Extensions to order print products in Photos after September 30, 2018 - Apple Support

To give users access to a broader variety of print services, macOS High Sierra introduced Photos Project Extensions which integrate directly with the Photos app. In macOS Mojave, Photos will transition entirely to Photos Project Extensions. Users will be able to print and update their existing projects using these new services, and will gain access to a broader variety of options.

For final purchases using the existing built-in service, place your orders by September 30, 2018.

And this looks very final to me.

The current state is a disaster and a bad timing, just when so many users want to get their annual Christmas presents (calendars, books) printed.

Use the feedback form to let Apple know: Apple - Photos - Feedback

Perhaps it will help. Apple once brought the print services back for iPhoto '11 (but not for Aperture), after they had been discontinued for several months. So there is (a little) hope.