+1 yes, this is essential. I think we have some sort of support for PubSub in our Android client as kind of an add-on module or something. Not sure it this makes sense for iOS though. So, yes PubSub support for library and then some specific application in the client.

Protocol support is implemented and documentation for feature is added in code. Still need to test implementation and add example of usage to documentation.

Feature is implemented tested and documentation is updated.

As for features which will make use of PubSub in iOS messenger, I think VCard4 and User Avatar are good features to implement as an example of usage. For that I created separate tasks #4613, #4614 and #4615