Traditional footnotes order is asterisk*, dagger†, and double dagger‡, with some medievalist using a tripple dagger after that. I use a double asterisk** (I deliberately use two ASCII asterisks** instead of a double asterisk glyph⁑) as my level two, so I have *, **, † and ‡ as four levels.
I just prefer having two asterisks that way, I think that looks fine enough and it keeps it ASCII clean and it introduces a li’l hurdle for me to make me less likely to have three or four footnotes in what’s supposed to be a quick message.
If I need more than four I’ll just put them on a second page or use numbers instead.
This mostly goes for chat (like IRC and XMPP) and email and on Fedi since in books and HTML I can use sidenotes instead which don’t need any superscript glyphs. And in the contexts where I do use this stuff, I don’t bother with trying to superscript them or even mark them up in any kind of semantic or style, and same goes for the footnote text itself. I just literally jam the note character inline with the text and start the “footnote” with that character followed by a colon.
This is a li’l wonky accessibility-wise but these protocols are already limited enough in what they can do.