I have installed various versions of Mathematica using the standard installer (the most recent, the V12.0).
I can find the path to the older versions of Mathematica. For example. in bash
ls -l $ (which Mathematica)
lrwxrwxrwx 1 root root 59 17 Apr 17:46 / usr / local / bin / Mathematica -> /usr/local/Wolfram/Mathematica/12.0/Executables/Mathematica
From that, I was able to find earlier versions installed in parallel directories (with "12.0" replaced by the version number).
For example, I can launch V11.1 with