Any sufficiently complicated library management system contains an ad-hoc, informally-specified, bug-ridden, inconsistent implementation of half of the Dewey System [2].
[1] https://en.wikipedia.org/wiki/Greenspun%27s_tenth_rule
[2] https://en.wikipedia.org/wiki/Dewey_Decimal_Classification
It's also not informally specified. The shared link is literally the specification document. It's written in a kinds of informal style, sure, but that's a different kind of informal - Greenspun's informal means "not written down at all".
https://johnnydecimal.com/00-09-site-administration/01-about...
Uncharitable. The fact that this is called "Johnny Decimal" is a nod to Dewey Decimal in the first place