From what I'm seeing looking at PHP7.2+ is that there will likely be very little compatible with PHP5; at some point there will need to be a final version for PHP5 support, but we're not 100% sure when that line will be drawn.
If you look at the previous versioning you'll see that we've done this in the past for PHP4, PHP5, PHP5.3, PHP5.4, and now for PHP7 and PHP7.2+ support; its just how it is, newer functions that're introduced require newer PHP builds.
Keeping up with evolving standards is a significant amount of work and is done in phases, working towards PHP7 support is what we've been doing for several years now and involves a lot of steps since we require our shipped libraries to support the systems that you run UBB.threads on. PHPMailer6 for example will only run on PHP 5.5+, PHPMailer 5.2 (which we currently ship) however will not run on PHP7.1+.
