The most useful Domains read like a briefing for a new hire, not a full architecture document — the handful of things that would otherwise take someone weeks to figure out on their own: which convention this codebase actually follows, which shortcut is a trap, which file owns which decision.
Keep it current the same way you'd keep a shared README current — when an agent (or you) discovers something that would have saved time to know upfront, it belongs in the Domain, not just in that one conversation's history. A Memory is the lighter-weight version of the same instinct, for a single note rather than a whole rewrite.
A Domain that's grown stale is still better than none, but a five-minute pass every so often — trimming what's no longer true, adding what keeps coming up — is what separates a Domain that quietly saves you time from one everyone works around.