This isn't really like subtree it's like submodules, but for specific files. This could really be useful, you may want to keep track of a specific file in a repo but not need it as a submodule.
Yes, basically this. I think git-fetch-file has been used in some larger projects based on Googling it, so it seems to be different in that it's been vetted by other orgs, but like Mullvad, theirs is built by a corporation.
There's a whole umbrella of similar-ish utilities. The one that I think does almost the same thing is: https://github.com/mullvad/git-carbon