This isn't the formal Qt 5.9 beta but rather a snapshot to encourage testing and weed out any potential blocker bugs ahead of the formal 5.9 beta release in the near future.
Qt 5.9 beta snapshot details available via this mailing list post.
Also today is a new Qt blog post concerning the shader binary caching in Qt 5.9. Qt 5.9 is featuring GL_ARB_get_program_binary support for allowing shader binary caching by Qt itself to speed-up caching when making use of OpenGL in Qt. We've talked a lot about the Mesa/Gallium3D shader caching support that's landed while this is an effort to do it application-side. ARB_get_program_binary is mandated by OpenGL 4.1, but Mesa doesn't expose any binary formats for the extension, so there it's left up to the new driver-side shader caching support in RadeonSI/R600g/Nouveau.
Details on other Qt 5.9 features can be found from this earlier article.