In theory, if you were worried about this, I think you could get around that problem by just constantly send fixed sized encrypted messages large enough to hold any potential notifications. When a notification needs to get sent, you add it to the next outgoing encrypted message. Of course, now the delay for notifications is now coupled to the rate of these messages being sent.
The messages can still be traced to the client picking them up. Cut their networking and the client doesn’t receive the messages anymore… this shows up as a bounced message.
You can accumulate encrypted messages at random urls instead
If nobody in the chat room can know that you lost connectivity, that means you can't do real time chat and might as well use email instead.
Note that a similar thing happens with any two-way conversation; if an immediate reply is expected and there's no reply, it's not a conversation anymore. (For more possible reasons, though.)
Disappearances can't have good error messages if you want plausible deniability. This will make any UI much less user-friendly.
