The location for System-wide and User-only installation depend on Windows-managed locations, which means that they will never require a user to select the correct location. Therefore it is best to hide the page in all modes except for Portable, which should still offer a path to extract to.
Thanks to the PR by @abcdw (https://github.com/obsproject/obs-studio/pull/4067) we can support 'local user' installations. These will allow end users to install StreamFX without requiring Administrator rights, or learning how to use OBS Studio in portable mode.
With OBS Studio 0.15.0 came a better location for plugins to install to, which solves many of the current issues. While this location still requires Administrator rights to write to, it is a much safer location than writing directly into the OBS Studio installation directory.