Clip Skip is only for SD1.x

There's really not much to add to this.
The CLIP used in SD 1.x has 12 layers, and Clip Skip stops before the last, last but two, etc. This can get better prompt adherence, ymmv.
Other SD models and Flux use a different CLIP (OpenClip from OpenYouKnowWhat) that has no layers.

Quote from the help popup for the Stop Clip at Layer option in SwarmUI:
Also known as 'CLIP Skip'. Default CLIP Skip is -1 for SDv1, some models prefer -2.
SDv2, SDXL, and beyond do not need this set ever.
The guy who wrote this worked for Stability AI, developing (Stable)SwarmUI.