Seems like anything pointing to netlify.app is down.
If your DNS records point to something.netlify.app, you can change it to the A record shown in your account. In ours, it’s 75.2.60.5.
That immediately solved our issue for now (we have low TTL on the DNS records), although we will eventually move back to netlify.app.
For apex domains, using ALIAS to apex-loadbalancer.netlify.com may also work but we haven’t tested that yet.
As an additional data point, this seems to only affect domains who use a third-party DNS provider.
Some of our domains domains use Netlify DNS, and they do not seem to be affected, even when the records in the UI point to something.netlify.app. But the domains that use third-party DNS providers are affected.
I am a Netlify user, not an employee, so I have exactly the same information the general public does. This is not official advice.