You can test the server for compatibility with the UBB.threads v7.6.0+ System Requirement Test
(most servers will be compatible, but if there are items such as suhosin with default settings or a small max_input_vars value there could be issues, which this tool checks).
The last minor release was v7.5.9, which as you noted came out on January 27th, 2015; the next major release, v7.6.0
, is currently in development. The alpha v7.6.0 release is currently what is running at UBBDev (as the team at UBBDev (myself and id242) are developing this release), with UBBCentral running its own unique preview build. The upcoming release is planned to be released in early 2016.
As for PHP v7.0 compatibility, it was released on December 3rd, 2015, and as v7.6.0 is a current work in progress we have not done any tests on a server with PHP7; and, until it becomes wide spread as an option on shared hosts, testing will be limited to short term goals versus changing coding for systems that're not yet widespread and may limit compatibility with current supported versions (which currently is v5.3 as a minimum).
Any build after v7.5.8 is considered secure, as this build was primarily released as a security release.