I believe they used to do `--depth` but GitHub complained because their systems weren't optimized for that and it ended being cheaper for the servers to send you the whole thing (much easier to figure out what refs you needed, or something along those lines).
I don't know anything about how git servers are implemented, but this is super weird to me. Why couldn't you basically just copy the files (excluding .git), then do `git init`, set the upstream remote, and fetch only the most recent commit? I'm guessing it wouldn't work _exactly_ like that because the git cli is convoluted and nobody remembers the exact semantics of every flag, but it seems surprising that there aren't any possible sequences of commands that do that. Even if they implemented it on server side by just making a "fake" client locally doing this, and then sending the tarball of the repo with just that commit, it would be way more efficient than making clients download the entire history.
I wasn't super sure how subsequent `git pull` works with a shallow clone, but if it works the same, then such an obvious fix makes the invention of that API solution even more painful
IIRC, the only thing that will potentially need to be changed is manually fetching the tags, since those won't be brought in. You can just do `git fetch --tags` to get all of the tag metadata though (without actually needing to pull the actual commits they reference; it just gives you the ability to look up a commit hash with a tag so that you can fetch it later).
Or even just `clone` with `--depth=1`? Just because the history exists doesn't mean you have to fetch it.