Afaik, Google doesn't publish public PMs for non-paid offerings, so youtube doesn't get a public pm.
For the August outage, I believe there was a public pm. That said I can't find it now (I think there was some link rot somewhere, and I've escalated about that).
- YouTube outage this November 2020
- August 2020 outage of Google Suite including Gmail
in both cases no postmortems were published