Introducing the Total Minor Versions chart. Historical chart displaying the amount of minor versions for each major WordPress version. Source
The chart is based on release dates of all the minor WordPress versions, it sums the total number of minor versions for each major versions ever released.
In the past, with every new WordPress release, the support for older versions immediately stopped. But as a major part WordPress users didn’t update their core software to the latest version, there has been a rise in attack targeting sites using older WordPress versions.
With the introduction of automatic background updates in WordPress 3.7, we see a rise of maintenance and security updates for older versions. Each security issue found in core is applied on all the previous major versions back to WordPress 3.7 by releasing minor versions.
As mentioned above, updates for minor versions are applied automatically. This made the whole eco-system more secure as older versions are updated seamlessly. As a result, you can say that since 2013 the auto-updates feature changed the old policy of not supporting older versions. Without officially announcing that WordPress started supporting