As far as I know, there wasn't a public postmortem for the Oct 16, 2018 YouTube outage. Based on that, I'm not expecting one for this outage.
"Red" incidents on the GCP status dashboard, like this one, will typically have public incident reports.
Disclaimer: I used to work in GCP Support, but no longer do.
YouTube ain't a Gsuite service with SLA. So this is much more likely to have a public report. Whether it's detailed or not is another thing.
Also it's probably due to the underlying GAE or GCS issue is my guess.