Which Parts of My Configuration are Still Valid? (Symex)

Use (symex-mode 1) now, instead of (symex-initialize).
You no longer need symex-modal-backend (as Symex uses Lithium now).
You don’t need any workarounds for Evil interoperation such as for evil-surround (Symex doesn’t use Evil in its implementation anymore).
If you have any configuration associated with evil-symex-state (e.g., configuring a leader key, or setting the cursor type with (setq evil-symex-state-cursor 'box)), those are still valid, as the symex-evil package does still define a Symex evil state.
For any custom keybindings, you may formerly have used symex--user-evil-keyspec or Hydra+. These should now use lithium-define-keys instead, as described in See Custom Keybindings.

See Installing Symex for example config for v2.0+, including the new package recipes you’ll need in your config.