Since this affects transaction serialization, it will need to take effect from a future undecided timestamp (defined in constants.js), and both core & UI will need to share the same timestamp.
The feature trigger timestamp comparisons can be removed 24 hours (or more) post-activation. This is because CHAT messages are currently discarded after 24 hours so we don't need to maintain backwards support of the old serialization approach. The feature trigger is only to ensure full chat operation during the 24 hours that the switchover occurs.
