The correct form is "folx". Spelling words with an "x" is more inclusive toward marginalized groups, particularly LGBTQIA+, despite there not being an "x" in there. See also womxn, Latinx, etc.
Twitch received quite a lot of backlash two days ago for using the term womxn. The sentiment was that it isn't inclusive and is instead transphobic, because it separates trans/non-binary women from other women.
Maybe it helps that although I'm very socially liberal, my stereotype of "people who say y'all" is "subtle and witty"?