phpBB provides the base software, including 2 styles, and a small number of official extensions. Aside from that, any and all other MODs, extensions, styles, etc. are made by unaffiliated, 3rd parties. That code does not belong to the phpBB team, nor is it their responsibility to keep track of and maintain it. Even styles and extensions that are reviewed and tested by the respective teams is only reviewed and tested for 1:1 version compatability. That is to say, an extension that releases its version 1.0.0 has that version tested against the latest version of phpBB at the time of that release. The phpBB teams are not responsible for making sure those extensions remain compatible with future versions of phpBB, nor that future versions of the extension are compatible with past versions of phpBB.
Unfortunately, aside from those who do freelance extension and style development, everyone working with phpBB is a volunteer, including 3rd party contributors. This means that it is entirely up to the contributor whether they want to continue to support any given contribution that they have made. That has the side effect of many extensions and styles falling by the wayside when the original author loses interest or no longer has the time or energy to devote to maintaining the contribution.
Sure, it would be great if there was a group of people working concurrently to keep various styles and extensions working with phpBB, but that presents many, many challenges (namely, who then owns the code and the direction for the code?), and is also rather infeasible. This isn't even done for other open source projects, such as WordPress and Drupal, whose communities frankly dwarf the phpBB development community.
One of phpBB's core principals, at least one that was such when I was a team member, was to keep the core codebase relatively lightweight and easily extensible. Perhaps a shortcoming of that philosophy is that 3rd party contributions are always going to end up falling behind the core product for various reasons. That being said, it is also a strength of the project, because people aren't limited to what the phpBB dev team is able to fit into a release. Nearly anything someone can think of can be done with some patience and thought.
And in the end, if an extension isn't compatible with the latest version of phpBB or still hasn't been ported over from its MOD predecessor, then maybe it isn't time to update yet. No one is forcing you to update, so unless there are specific bug fixes or security patches that you need (and particularly ones that can't be backported) then there is likely no reason to jump the gun and upgrade right away. It is likely that extensions will be updated eventually, either by the original author or another member who has benefited from the extension and wants to keep it alive.
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