(In reply to steve from comment #2)
Please see how this lack-of-support impacts functionality in related projects. It would be a shame if the only browser that is able to perform is Meta.
IIUC this kind of implies that we would want to use this mainly for performance improvements, not only for enhanced security. In fact the spec says about consequences:
In terms of observable effects, this means that attempting to relax the same-origin restriction using document.domain will instead do nothing, and it will not be possible to send WebAssembly.Module objects to cross-origin Documents (even if they are same site).
Behind the scenes, this isolation can allow user agents to allocate implementation-specific resources corresponding to agent clusters, such as processes or threads, more efficiently.
So supporting this for the mandatory part might be relatively easy (to be fair, I ignore what it really takes), but having also the resource benefits might be harder. Would it be worth doing one without the other?