@lumi
I think the matrix folk were pushing for a modified version that didn't require retaining ephemeral key data as long for out of order messages. It looks like there are still people working on an ietf draft. Not sure if they're the same effort.
mailarchive.ietf.org/arch/msg/mls/rqftoNl-rLXK53JYFgc3CcXpQQU/