I've released MKVToolNix 6.1.0. It's mostly a bug fix release with only small new features but one big enhancement: memory usage has been reduced severely. This is especially more noticeable the more video and subtitle tracks you're muxing, e.g. with Blu-ray discs.
For package maintainers: as libEBML 1.3.0 and libMatroska 1.4.0 have been released in March this release of MKVToolNix can be compiled with external versions of both libraries again. The source package still contains both libraries, but they're only used as fallbacks.