It can only apply shader(s) to the current frame I think. To produce the crt ghosting you'd probably need access to the previous frame (not an expert).
I've tried the shaders in the following repo with ghostty. They definitely work. I ended up keeping a cursor trail shader. https://github.com/0xhckr/ghostty-shaders
I've tried the shaders in the following repo with ghostty. They definitely work. I ended up keeping a cursor trail shader. https://github.com/0xhckr/ghostty-shaders