Not being funny but you clearly didn't read the thread.
2.1 has not removed the calendar. It is still a core feature.
The point being made was that it has been fiercely debated both ways to remove it in a future version and make it a plugin - and at the same time make the plugin system much more reliable so it would be much saner to actually make it a plugin if it were actually done (and it would be available with 3.0 for that reason).
The calendar is a strange beast. It is used by a tiny percentage of the SMF userbase... but those that do, make heavy use of it. It is a maintenance nightmare, it has a number of bugs in it, it is also a performance hog (more than it needs to be) and a plugin with proper importer would make a lot of sense for various reasons.