The link in the aforementioned notification when you don't have a key is old.ppy.sh/p/api whereas the actual link to get an API key is osu.ppy.sh/p/api
I believe the API link is still correct. It's a little complicated - you have to log in to the old website first; only then you can get to the API page without it redirecting you somewhere else.
The link in the aforementioned notification when you don't have a key is old.ppy.sh/p/api whereas the actual link to get an API key is osu.ppy.sh/p/api