If I use homebrew or macports to install the latest versions of these languages, are their runtimes included? Just asking.
It's not a big deal for users of Homebrew or other package managers.
If the Homebrew team aren’t already busting ass to rewrite brew in Objective-C, and negotiating to get it distributed on AppStore or—ideally—included in Xcode as standard, then they’ll have no-one but themselves to blame when they utterly fail to capitalize on the massive market opportunity Apple has handed them here. Believe me, porting brew is a nothing price and the simplest step by far.
This is a once-in-a-product’s-lifetime chance for a proven FOSS platform not only to grab ten million new users but to influence a global platform vendor’s direction too. Fail to seize this opportunity or blow it execution, and you won’t get another (I speak from painful experience here).
A standard installer package would be much better.
Installing via a shell command has nothing to do with the (in)ability to verify signatures. You aren’t more protected running a random installer package than an auditable shell script from the official Homebrew repo. Also, Homebrew supports more installation methods including cloning the git repo by yourself or un-taring an archive.
Homebrew circumvents all the protections built into macOS (like Gatekeeper / Xprotect etc) for convenience.
I don't think this method for distributing software has much of a future.
One good upside from this deprecation is that lots of scripts naively containing "#!/bin/python" and such without invoking them through /usr/bin/env will stop working on macOS, which is good.
That being said, this is a good thing. By pinning the interpreters to ancient versions Apple is maintaining this legacy code that is often replaced by homebrew/Macports anyway.