Most postmortems produce a timeline, a root cause, and a list of action items that nobody follows up on. The template matters less than the process — specifically, who owns each action item and when it's reviewed.
The section that's always missing
Every postmortem template includes impact and root cause. Almost none include 'what we learned about our system that we didn't know before.' That's the section that converts incidents into compounding institutional knowledge rather than a forensic report that gets filed and forgotten.