The feature was lost in the initial release of Pages 5, but was brought back in in iWork 5.2.
You can choose the shortcuts F1 through F8, and it appears to be a per-document setting. I suspect the reason it has to be per-document is that the styles themselves are part of the document. You may also be able to apply shortcuts to a template, so that new documents based on it get the shortcuts, if you use the same templates/styles a lot.
The UI in Pages 5.2.2:
I'm not entirely sure why, but Apple have done similar things in a few of their applications, with major updates (e.g. 4.x to 5.x) seeing a bunch of features disappear with a simplified UI, and then some of the lost features come back in minor subsequent updates.