I know that Sans Display has been deprecated and tossed in the bin, but is Serif Display also deprecated, or just active-but-quiet? Noto Serif Display v2.009 is listed under Releases here, and the files are listed on the Dashboard page. ...

I'm not sure they were, though; that's the problem. The "display" version was not exactly optical size and not exactly contrast, and was a different implementation between serif and sans. It didn't really fit into any of the models of font variation that we support elsewhere. So they need re-thinking, harmonising, and re-drawing.

You can consider them to be deprecated.