I talked to Engineering about the latest version being available on Maven Central just a couple weeks ago. I was told it's on their todo list, so hopefully soon. You could always file an enhancement request in the bugzilla for this to happen.
The 4.4 runtime does not work from maven central for xslx generation. It turns out that the download of runtime from eclipse has a different runtime jar file, some 9MB more than what is put into the maven central. Whatever the difference is it prevents xslx generation.
[tl; dr: having the apache poi jars embedded in the runtime jar trips up people whose environment needs different versions of poi]
Having two different sources for different runtimes is clearly silly, so I think the intended long-term solution is to have the runtime bundle specify dependencies on the relevant non-BIRT components rather than just piling them in itself. But for now it sounds like you want the version from download.e.o.
I am not adverse and agree that the poi should be removed from the birt runtime. However with the version from central I run into the issue that it will not create an xlsx file. All other files work. When I switch and use the library from the download version it does work. I know that I should look through and figure out what is missing or where the mismatch is, but thought that it should be fixed overall. I like the idea of having central manage this as it makes it easier to work across different developers. But it doesn't work out of the box so to speak. Maybe I will take time and figure out where it went wrong and why but maybe somebody who knows more will figure it out for me!