Observed in installed Markdown 3.10.2 and markdown-it-py 4.0.0
Python-Markdown markdown/inlinepatterns.py: build_inlinepatterns registers LINE_BREAK_RE with SubstituteTagInlineProcessor, tag br.
LINE_BREAK_RE = '  \\n'
markdown-it-py markdown_it/renderer.py: RendererHTML.softbreak
    def softbreak(
        self, tokens: Sequence[Token], idx: int, options: OptionsDict, env: EnvType
    ) -> str:
        return (
            ("<br />\n" if options.xhtmlOut else "<br>\n") if options.breaks else "\n"
        )