The whole point of Unison is to allow sync of file trees between different computers running different versions of an operating systems or even entirely different operating systems. Because not all systems can have the one same version of Unison at least one preferably two older good versions need to be kept. That is supposed to be what the package unison-all is about! The unison package gives you the latest, the unison-all package makes Unison useful! In stretch unison-all should include at least 2.40 and perhaps 2.32. Please!