As I understood it, scripting languages will still be available to the user, they just won’t have ancient versions preinstalled. Of course that also means you can’t rely on them being installed at all as a developer.
Which means you should start bundling the stuff you need to run your app, and, they’d like you not to use the same ancient batteries as they have previously made available, because, well they are ancient.
Ok, I guess it's worded vaguely enough to allow for the possibility of optional installation ("by default"), even though they don't outright promise that they'll offer a way to install it. (I interpreted it in the same way as "macOS doesn't include Photoshop by default".) Having it come from the app store doesn't seem totally appropriate since it would need to do more than dump a bundle into /Applications, like making some /usr/local/bin symlinks. Maybe they'll allow themselves that exception.
"they’d like you not to use the same ancient batteries as they have previously made available, because, well they are ancient" --> Right, I couldn't figure out if that's all they meant. Valid point or not, it doesn't seem appropriate to make it in the context of those notes. If you're going to make me bundle the language anyway, don't also give me a lecture about which version to use. :)
I suspect they plan to distribute the scripting languages the same way they currently handle the command line developer tools package, where the OS offers to download and install the package the first time you try to run one of those tools.
I just don't see the win of that over supplying the thing with the OS in the first place, unless the commands to do it are a nascent versioned ecosystem along the lines of brew, apt, etc.. It seems like the whole point of not supplying them with the OS is to make the versions less tied down. If there's some opaque command to get an arbitrary version, that's not going to help anyone.
Which means you should start bundling the stuff you need to run your app, and, they’d like you not to use the same ancient batteries as they have previously made available, because, well they are ancient.