What is the 'entanglement horizon'?

The entanglement horizon is a conjectured speed limit in branchial space: a proposed bound on how fast entanglement (branchial correlation) can spread from one part of the multiway system to another, loosely mirroring how the speed of light caps the spread of influence in physical space. [conjecture] It is offered as an analogy and aspiration — the popular essays introduce it in barely a phrase — not as a measured quantity or a proven theorem. Whether such a bound really holds, and what would set its value, is left open in the model.