I wouldn't call KDL "established"; it's honestly pretty niche. It looks _similar_ to pre-existing configuration formats, but KDL is very picky about syntax. I.e.: single word string MUST be quoted.

In this same space, https://codeberg.org/emersion/scfg/ is pretty widely used. The curious thing about scfg is that it's not really a "new" format: the syntax described has been in use by different tools for decades. They just didn't call it scfg, and it's mostly the name itself that's new.