[2020] None of my mods are showing up in the Mods section of the options menu.

Sometimes logging out of Steam, shutting Steam down, restarting Steam and logging back in also does the trick.

Other times it can be an old broken mod that messes with the UI. Checking your mods against this list of broken and incompatible mods is always good to make sure you avoid broken stuff.

https://docs.google.com/spreadsheets/d/1mVFkj_7ij4FLzKs2QJaONNmb9Z-SRqUeG6xFGqEX1ew/edit#gid=0