The 1.0.x series is in strict maintenance mode and will not change build systems. This is targeted towards embedded use, as in projects which already embed the bzip2-1.0.6 sources and undoubtedly patch the build system. Right now this series, and the tagged 1.0.7 release, live in the sourceware repository for bzip2.
The 1.1.x series has Meson and CMake build systems, and a couple of extra changes to modernize the C code but which were not fit for the 1.0.7 release. This is targeted towards operating system distributions. This lives in the master branch of the gitlab repository for bzip2.
Distros and embedded users should start using bzip2-1.0.7 immediately. The patches they already have for the bzip2's traditional build system should still apply. The release includes bug fixes and security fixes that have accumulated over the years, including the new CVE-2019-12900.
Once 1.1.0 is released, distributions should be able to remove their patches to the build system and just start using Meson or CMake. You may want to monitor the 1.1.0 milestone — help is appreciated fixing the issues there so we can make the first release with the new build systems!