Yesterday, Google dropped Chrome version 50 into the stable channel for all users. The newest version of Chrome for Windows, Mac and Linux brought with it a number of new security tweaks and features, some of which were made by outside researchers who were paid quite handsomely for the help, as well as better preloading with declarative support and a ton of improvements to push notifications. Buried deeper in the change log, however, was mention of a promised change that Google laid out the specifics of a while back. The newest version of Chrome no longer supports Windows XP and Vista, or OS X Versions 10.8 or older.
Older versions of Chrome are still kosher on the older OSes, but they won't be receiving any further updates. Attempts to run the newest Chrome on these older systems will simply result in nothing much happening, forcing users to go for older versions of Chrome or newer operating systems. In some cases, users may simply have to upgrade their hardware; many old Mac systems or PCs that shipped with XP and Vista, if they were low-end or mid-range at the time of their release, just don't have the horsepower to run Windows 7 or OS X 10.9, the minimum requirements for Chrome 50. Since some Linux systems are long-term supported for up to five years, such as Ubuntu 14.04, or have a running release that updates in pieces, like Puppy Linux or Arch, Linux update compatibility is a bit more of a sticky situation.
Older versions of Chrome on older OSes may begin to see functionality loss in the near future, such as notifications, Google sign-in and cross-device sync. This was not specified in the release notes or any official Google media, but commonly happens as newer software moves in other directions and uses new APIs for old functions. It is entirely possible, of course, that the older applications on the older operating systems could just continue working by shoehorned backend tweaks, but with Google's recent focus on security including the ousting of outdated software and systems, it's not hard to imagine them leaving older versions of Chrome in the dust.