It looks like no one "owns" algo.graph and there are no releases on Maven Central? /cc @alexmiller was the last person to commit any changes there (four years ago).

I don't see any other reference or ticket about this change and was wondering if there was something I was missing. And whether this change was safe to go ahead with given that it was safe for so long.

IIRC the previous behaviour was considered a bug

Which seems right, but the only way to use this before was to do it the "incorrect way", so I wonder if it has taken root in a few code bases out there. It came up in our development when switching to CLJS 1.9. We luckily didn't use it in many places but it seems like this could be bad since the buggy way was in fact the way to get it to work.

Quite a few libraries "in the wild" have been broken by the specs introduced in Clojure 1.9 Alphas -- but in every case the library has just fixed their code.

There's a wiki page listing libraries that had bad syntax, allowed prior to 1.9, and it shows the version of the library that includes the fix...

Here it is https://dev.clojure.org/display/design/Errors+found+with+core+specs