Is it mandatory to upgrade to Redactor v3, or can I keep using v2 if I want to?

You can keep using Redactor 2 if you'd prefer, though we do recommend checking out the new features that make v3 much more pleasant to use. 

The package manager in MODX will allow you to choose the version you want to install. Simply don't choose Redactor 3 to stay on Redactor 2. Note that Redactor 2 is end-of-life and will no longer receive updates or support since July 14th, 2020.