There is very limited technical documentation on xthin so it is hard for the community to accept and test it. If Bitcoin were to implement xthin (or an alternative proposal such as compact blocks) it would be much better to release outside of the block increase. again the main contention is the mechanism for change that the Bitcoin unlimited enthusiasts want to impose, a contentious hard fork

And to clarify, the software may have been released (in Bitcoin unlimited's release), but is not used by the Bitcoin network as a whole. Very few nodes are running this implementation. And to many is untested/unproven