what is a post mortem?

That's explained here: https://twitter.com/devtober/status/1189612484900077570?s=20

It's a document where you reflect the devtober :-)
Here are my examples from past devtobers:
'19: https://el-falso.itch.io/castle-attack-prototype/devlog/107242/post-mortem
'20: https://itch.io/jam/devtober-2020/topic/994257/el-falsos-devlog?after=20#post-21...