[jn]

The feature is opt-in, requires a somewhat formal process to turn on, and all users will be notified "via Slack that the feature is active and that their private messages are subject to export". They don't make it clear if it'll be made sufficiently obvious to a new user who joins after it's turned on, but I'd guess so.

Also quite nice that "Compliance Exports are not retroactive and do not apply to past private conversations before the feature is enabled"