A useful postmortem has a timeline with times, the impact (who was affected and for how long), the contributing causes, what went well and what didn’t, and a short list of actions with owners and dates.
Postmortems are blameless: they ask how the system and the process allowed the mistake, not who made it. People who fear blame leave out details, and the details are where the fixes are.
Publishing a short version builds trust. Customers who were affected want to know the outage was understood and what changes; a clear public review often does more for trust than a perfect uptime record.