This example overrides the default HTML renderer to produce only header and footer, but omit the document body. That's an equivalent of using `HTML_OMIT_CONTENTS` with Blackfriday v1.

omitcontents.go

package main

import (
	"fmt"
	"io"

	"github.com/russross/blackfriday.v2"
)

// OmitContentsRenderer is a thing.
type OmitContentsRenderer struct {
	defaultR       *blackfriday.HTMLRenderer
	doneWithHeader bool
}

// RenderHeader writes HTML document preamble and TOC if requested.
func (r *OmitContentsRenderer) RenderHeader(w io.Writer, ast *blackfriday.Node) {
	r.defaultR.RenderHeader(w, ast)
	r.doneWithHeader = true
}

// RenderFooter writes HTML document footer.
func (r *OmitContentsRenderer) RenderFooter(w io.Writer, ast *blackfriday.Node) {
	r.defaultR.RenderFooter(w, ast)
}

// RenderNode is RenderNode.
func (r *OmitContentsRenderer) RenderNode(w io.Writer, node *blackfriday.Node, entering bool) blackfriday.WalkStatus {
	if r.doneWithHeader {
		return blackfriday.Terminate
	}
	return r.defaultR.RenderNode(w, node, entering)
}

func main() {
	md := "# Heading1\n\nIntroduction.\n\n## Heading2\n\nConclusions."
	defaultR := blackfriday.NewHTMLRenderer(blackfriday.HTMLRendererParameters{
		Flags: blackfriday.TOC,
	})
	or := OmitContentsRenderer{defaultR: defaultR}
	html := blackfriday.Run([]byte(md), blackfriday.WithRenderer(&or))
	fmt.Println(string(html))
}