Plex does - if you give them money. A lot of app packages offer customizable icons provided you give them money for the privilege.

I disagree with this approach, and vendors that lock such changes down. If a user wants to replace every single app icon with a PNG or SVG of their choosing, that should be permissible at the OS-level. Users should always have the final say over their interface choices, and corporate or software-maker changes regarding aesthetics/interfaces should never override what the user has chosen for themselves.

Right now there's strong overlap in interfaces and experiences that make this difficult, if not impossible to execute on. Separating the two again is critical for computing to be accessible to all, as is maintaining a consistent experience throughout interface changes.