Since I have changed Qt toolchain into Qt 5.12, it supports 10.12 minimum version https://doc-snapshots.qt.io/qt5-5.12/macos.html
. Engine doesn't uses Qt: it uses SDL2 only. Editor, Calibrator, Maintainer, MusPlayer are using Qt, and they are was affected.
To get back support for older macOS versions I need to make using of older Qt toolchains, that will need some time to let me compile from source the compatible static Qt toolchain...Added after 2 minutes 37 seconds:
Okay, to avoid confusion, I have changed minimal version to 10.12 on Laboratory page. If you really need to use old ElCapitan and you can't upgrade your system, I'll try to build older Qt toolchain and will produce a separate build that will support older macOS versions.