/usr/lib/mono/4.0/Mono.Debugger.Soft.dll is in libmono-debugger-soft4.0-cil 2.10.8.1-1ubuntu2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/mono/4.0/../gac/Mono.Debugger.Soft/4.0.0.0__0738eb9f132ed756/Mono.Debugger.Soft.dll