Having the major version in sys.platform only for a few selected platforms doesn't seem too consistent for me.

If linux 4 will introduce major changes, will we have to look for the major version of freebsd in sys.platform and for the linux major version elsewhere?

That said, removing the major version for the other platforms might introduce unnecessary breakage, but if it's ever going to happen, I think it's better to do it now for all the platforms and not waiting for the experts to speak and change it in later versions, adding even more inconsistency.