I'm surprised about its disappearance, as well. My guess is that whatever Apple found to be unacceptable in the first version (which led to neutering the app) may not have been fully corrected in the second version. Or, as Marilyn suggests, it may just be a temporary elimination to fix problems or add features. Revisions for iOS 5 may well be the issue. I have noticed that Xsysinfo appears to "crash" frequently since the iOS 5 update. Not sure if that is actually the case or whether the use of the task killer just kills the Xsysinfo app when it's done its job. In any event, that, too, may be the source of pulling it.
Have to admit that (as I may have mentioned in a previous post) as an iPad 2 owner I've not found Xsysinfo's task killing feature to be much more effective than those "close door" buttons on elevators that mollify the occupants but don't accomplish anything. YMMV if you're an iPad 1 owner with only half as much actual memory to use. (God, I hate Apples confusing reference to "memory" when they mean storage rather than RAM.)
In general, Apple's (again misleading) approach to "multi-tasking" should not require the use of a task killer. (That's actually true in the Android world, as well, but that's another discussion.) However, I suspect that as applications have been added and rewritten to take advantage of the 500K RAM available in the iPad 2 and as people have simply added more and more apps to their iPad 1's, the memory management in the iPad 1 may not be as successful as it was when the original iPad was introduced. Thus, apps like Xsysinfo may be more beneficial for iPad 1 owners than for those who have iPad 2's.
On a related topic, if I'm not mistaken (and I may be) I think the price of "Sys Activity" has been reduced to $.99 from $1.99. I haven't used it but it claims to have the same functionality as Xsysinfo. So that remains an alternative for those who are OCD about freeing memory. (Have to admit I'm often one of those folks though I'm not convinced it's worthwhile.)