macOS prevents opening text files with URLs if they lack a .txt extension

Your links experiment gives us the biggest clue. 

You renamed links to links.txt, changed nothing inside the file, and Finder immediately allowed it to open in Emacs. 

That points to the way Finder identifies the file, rather than anything contained in the URLs. There is also no evidence that Google Docs links have anything to do with it.

The fact that you can open the files from Terminal is useful too. It suggests that the files themselves are fine; the problem is more likely to be how Finder decides which application should handle them.

For an individual file, Control-click it in Finder and choose Open With >> Emacs. If Emacs isn't listed, choose Open With >> Other… and select it. Apple describes this here: Choose an app to open a file on Mac

You can also bypass Finder with Terminal: open -a Emacs "/path/to/links"

For files that are plain text, adding .txt is a perfectly reasonable workaround because it explicitly tells Finder what kind of file it is. 

Apple explains filename extensions here: Show or hide filename extensions on Mac

If Finder recognizes a file type, you can make Emacs the default for that type through Get Info >> Open with >> Emacs >> Change All. 

Unfortunately, that is not a general “open every extensionless file as text” setting. macOS does not have a preference that says, in effect, “I created this file, so let me open it regardless of what I called it.”

So, for your existing extensionless text files, Open With >> Emacs, open -a Emacs, or adding .txt are the straightforward choices. 

If you want Finder to automatically recognize your extensionless files as text, we would need to look at how Finder is identifying one of those files before changing anything at the system level.

I would not change any macOS security settings based on this evidence. 

Nothing you have described suggests that the URLs, Google Docs, or the fact that you created the files are being treated as a security problem.

Ellarsee wrote:

I eventually changed links to links.txt

No other change to the contents of the file, and macos let me open it.

This is pretty braindead, judging the behavior of a file on the suffix, rather than the content.

The problem is neither the extension nor the content. It's the software. A data file on disk can't hurt anyone. It's only when it gets loaded into some app that it could perform some kind of malicious act. Different apps have different capabilities. Modern apps are expected to advertise those capabilities. If they don't do that, then the operating system won't automatically allow them to open documents.

That's what you're seeing here. It isn't so much that macOS isn't treating you like an adult. It's that macOS is treating emacs like the a a very old person that doesn't understand the risks of the modern world. Emacs can't draw a clock, so it can't drive or open files. If you are an adult, then you can be a caregiver for Emacs. You can hold Emacs's hand and manually open your documents from within Emacs. But it wouldn't be safe to let any of these kids running macOS today take over Emacs' care. These kids today just don't understand.