It was promised or expected for Snow Leopard apparently.
Wired: https://www.wired.com/2008/10/rumor-apple-s-finder-app-to-ge...
Ars Tecnica: https://arstechnica.com/civis/viewtopic.php?f=19&t=99890&sta...
Also a quick search found this guy referring to it in 2009:
"I have always wished that I could change both the font and spacing between both sections and items. I was really hoping this would be possible with the rewrite of finder in SL 10.6."
https://macmost.com/forum/resizing-the-sidebar-font-in-finde...