This sounds like it was a bit before my time but looks like there are some references to that FTP site on this HN thread here [0] and TLDP article here [1]. Though GitHub/GitLab/SourceForge.net are similar, I miss the flexibility and simplicity of FTP sites and mirrors (though I only experienced the tail-end of that era AFAIK.) I'm glad to see that there are many Slackware FTP mirrors still online [2] such as OSUOSL [3] and XMission Internet [4] but perhaps these are less like the FTP sites containing much more than packages and source code?

[0] https://news.ycombinator.com/item?id=28565745

[1] https://tldp.org/HOWTO/META-FAQ-2.html

[2] https://mirrors.slackware.com/mirrorlist/

[3] ftp://ftp.osuosl.org/pub/slackware

[4] ftp://mirrors.xmission.com/slackware/