"FEP-171b: Conversation Containers" finally has been published:
https://codeberg.org/fediverse/fep/src/branch/main/fep/171b/fep-171b.md
Conversation Containers are conceptually very similar to FEP-1b12: activities are sent to a conversation owner, who manages the conversation and synchronizes it between participants. Differences are mostly superficial and may disappear in the future.