Speaking as the person running it - introducing API keys would not be a big deal, we do this for a couple paid services already. But speaking as a person frequently wanting to download free stuff from somewhere, I absolutely hate having to "set up an account" just to download something once. I started that server well over a decade ago (long before I started the business that now houses it); the goal has always been first and foremost to make access to OSM data as straightforward as possible. I fear that having to register would deter many a legitimate user.

There’s a cheapish middle ground - generate unique URLs for each downloaded, which basically embeds a UUID “API” key.

You can paste it into a curl script, but now the endpoint can track it.

So not example.com/file.tgz but example.com/FCKGW-RHQQ2-YXRKT-8TG6W-2B7Q8/file.tgz