I'd be less worried about "forgetting" and forever worried about a manager just saying "you obviously forgot (something I just decided)". I mean it happens even when there's documentation I can pull up and say "well actually..", I'd hate to imagine otherwise.

I love email for this. That way I can always pull up the email and say, "this is what we decided." I follow up all meetings with email(s).

Yes, this is one of my pet peeves.

Pivotal R&D has struggled with maintaining context. For multi-megaSLOC codebases with many moving parts, organic memory begins to fall a bit short.

Different teams deal with this differently. Some write design notes, some keep architecture design records. But IMO it's one of our weaknesses.