This feels like a good idea in principle, but I can't shake the feeling that it just moves the goalposts one step away:

Now your app doesn't have direct access to your stripe/github/aws/whatever keys (which is good!) but you still need to have _some_ authentication against your proxy.

If you have a per-app authentication, then if your app's key leaks, then whoever uses it will be able to reach all the external services your app can, i.e. with one key you lose everything. On the other hand, if you have per-endpoint authentication, then you didn't really solve anything, you still have to manage X secrets.

Even worse, from the perspective of the team who owns and runs the proxy, chances are you are going to use per-app AND per-endpoint authentication, because this will allow you to revoke bad keys without breaking everyone else, etc.

What this really solves is subscription management for (big?) organisations. Now that you have a proxy, you only need a single key to talk to <external-service>, no need to have to manage subscriptions, user onboarding and offboarding, etc. You just need to negotiate ratelimits.