fill-paragraph-function is a variable defined in `fill.el'.
Its value is

  • This variable may be risky if used as a file-local variable.

Mode-specific function to fill a paragraph, or nil if there is none.
If the function returns nil, then `fill-paragraph' does its normal work.
A value of t means explicitly "do nothing special".
Note: This only affects `fill-paragraph' and not `fill-region'
nor `auto-fill-mode', so it is often better to use some other hook,
such as `fill-forward-paragraph-function'.