Yeah, mail is the primary source of this.

Once communication with my customers moved to teams. I've had a very hard time to find historical agreements and decisions.

I try very hard to create a robust system for ADR logging now. And not just for system architecture. But for all decisions and agreements in my projects and across changes.

Methinks the better solution here is to get better friends?

Well I don't think most people choose who they work with. Even if you like your team a lot, you might have a discussion with someone from another team or division, and that's where it's useful to have a good chat history haha.

Doesn't really work in an org with 100s of people and where emails are automatically deleted after 6 months.

I expect that some types of people (in middle management, especially) may see the lack of this as a good thing.

A certain type of person sees this as a feature, not a bug.