The following input markdown:

Test [^] link

[^]: https://github.com

Produces different output depending on how things are set up and which implementation you use:

• In pulldown-cmark with footnotes turned off, it thinks [^] is a link.
• In pulldown-cmark with footnotes turned on, it thinks [^] is plain text. That's probably a bug, since nobody else does that.
• Commonmark.JS (which does not support footnotes), thinks [^] is a link
• In pandoc commonmark (footnotes turned off), it thinks [^] is a link.
• In pandoc commonmark_x (footnotes turned on), it thinks [^] is a footnote.
• In pandoc flavored markdown (footnotes turned on, not commonmark) it thinks [^] is plain text. It's the only other implementation I know of that acts like this, and it's not a commonmark implementation.
• In GitHub Gist (footnotes turned on), it thinks [^] is a link.
• In markdown-it¹ normal mode (footnotes turned on), it thinks [^] is a link
• In markdown-it strict mode (footnotes turned off), it thinks [^] is a link
• In Hugo (easiest way I knew of to try goldmark, because I couldn't find an online dingus; footnotes are enabled), it thinks [^] is a link

The smart money is on making it act as a link.

"[^]:\n" -> [
  Start(Paragraph)
    Text(Boxed("[^]:"))
  End(Paragraph)
]

"[^]:\n" -> [
]

Pandoc {
    meta: {},
    blocks: [],
}