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.

Noto Latin, Greek, Cyrillic. Contribute to notofonts/latin-greek-cyrillic development by creating an account on GitHub.

Serif does not have a full build. Sans has a full build for messy historic reasons.

Once upon a time when Google Docs/Workspace was young, they requested
Noto Sans but they also wanted it to work seamlessly with documents in
Hindi without people needing to change fonts, so they asked for the
version of Noto Sans in Google Fonts to contain a Devanagari glyphset.
Now Workspace's support for other languages is better but once you've
released a font with some glyphs in it, documents will break if you
take them out, so it's got to stay. So for the exceptional case of Noto
Sans, "full"/"googlefonts" builds mean "we added a Devanagari core".