A software development lifecycle (SDLC) was once conceptually very simple. You bought a licence for a particular software version, which usually entitled you to support and some bug fixes, and when the next major version came out, you bought another licence for that one. It was very simple for the software makers as well; once a major version was released, work could ramp up full speed for the next one. Ongoing support for any version was likely to be discontinued one or two version releases later.
Today, such is no longer the case. The Internet has made users accustomed to the idea of free updates to software available via download. Support for any particular version of software has become impossibly long (Windows XP takes the crown for the longest continually-supported software at 14 years). As a result of these and other factors, various different SDLC models have arisen; some of which are traditional, others more suited to agile paradigms, and yet others that work well in the “free software” world. All the above now provides you sufficient background to explain why Joomla! is switching from its current SDLC model to a new one.
Because Joomla is an FOSS project, it is impossible for its support to be as comprehensive as that of Microsoft’s, for instance (at one stage, Microsoft was supporting 4 different OS releases). Hence, it is important that the SDLC chosen matches the needs of the Joomla project. It is equally important that everyone in the ecosystem is aware of the major ramifications of this new release strategy (using the [major].[minor].[patch] version numbering system):
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No more specific Long Term Support (LTS) versions
Previously, Joomla’s X.5 (i.e. 1.5, 2.5) releases were classified as LTS releases, which meant that they would be supported longer than any other minor releases (e.g. 3.1, 3.2, 3.4), and will continue to be supported even after newer minor versions have been released. There is no longer any such guarantee; the last point version will be the LTS version. All major releases (v1, v2, v3 etc.) will have 2 years in the active development phase, and a minimum of 2 years in the support phase after that. If a new minor version is released during the support phase, it resets the 2-year support phase clock. Only the latest minor release within each major version will be supported
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Faster and more regular minor releases
Adding features to Joomla could be time-consuming, depending on whether you got it in before the feature freeze cut-off. With less time passing between each minor release, this means developers no longer need to be as frantic as before, giving more time to regression test their features before inclusion in a minor release.
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Major releases will break compatibility with previous releases
While it isn’t guaranteed to happen all the time, major releases are the only releases where backward compatibility is intentionally discarded. This means that each time you upgrade Joomla (e.g. from version 3.x to version 4.x), be prepared for things to break.
For regular users of Joomla, what does this all mean? Generally speaking, it means that in order to ensure that you have a smooth updating and upgrading experience, you should always update to the latest minor release of your major version, and that you should upgrade Joomla to the next major version every 2 years or so.