The former symex-mode minor mode had a narrow focus on balancing parentheses in Lisp modes. As Symex now works with any language, this general name for such a specific mode was no longer appropriate, and so it was renamed to symex-lisp-mode. Formerly, setting up the ability to use Symex in any buffer required directly calling symex-initialize in user config. A new, global mode named symex-mode (replacing the former mode of the same name) was introduced in 2.0 to handle this in a standard way simply as (symex-mode 1).
For any buffer-specific configuration for which you might have used symex-mode hooks formerly, you should use symex-editing-mode instead. This is the local mode that is active in a buffer when you actually enter the modal UI. The next section summarizes the purpose of each Symex minor mode.