Why not make Notion the canonical system of record for this? Github is for code, Slack is for ephemeral chat. Anything that must persists should be stored in Notion (and you can link to Github references from within Notion).

Notion does solve this upto an extent. I think the problem I face with it is that it tries to be an "everything" app for teams. They've designed it in a generic fashion.

We have so much documentation in Notion for different parts of work: i.e. internal finance, support, marketing, sales, ad-hoc ideas docs. And then when dev and several parts of dev lifecycle are added to it, it looks like a clusterf*ck (excuse my language). Everything is in your face and navigation seems clunky when you organise things one under the other. And for some reason, editing docs feels like I'm typing on a remote server with 300ms delay (on my linux device, works smoothly on Mac).

This is a reason why I like Linear. An opinionated planning product.