For those using CiviCRM 4.6 in particular, you're running a version that only supports PHP 5.6, which is officially end of life at the end of December.
Any host that upgrades your PHP beyond 5.6 to say, 7.2, will cause CiviCRM to not work. Needless to say, you've extra incentive to upgrade.
What about users on CiviCRM 5.3, the version with the last security release applied? We'd encourage you to upgrade as well, though perhaps it feels less critical to most. I mean, it did receive the last security update (though it's no longer receiving bug fixes), right? To be clear though, the next security release could come at any time. That's the nature of security after all... you find a vulnerability, assess the risk, and it if warrants, you push out a fix asap. What happens when a new security update is released? Well, CiviCRM 5.3 won't receive the update and you'll be vulnerable if you don't upgrade.
December is here, and the days are moving fast. Older versions of CiviCRM are end of life, so we encourage you to have a plan to upgrade, whether to the lastest stable or to Extended Security Release (ESR). For those wanting to maintain the same version of CiviCRM for longer, ESR is the best option for you going forward. More info on ESR can be found here: https://civicrm.org/esr