While trying to understand more completely the usage of YAHOO.util.Connect (for PR-6840), I was looking at variables.js (thanks Basil for the pointer).
I was not able to configure the plugin correctly as the repeatable agent configuration was broken. It seems that the placeholder content is displayed but it's not parsed when sent to the server and there is no way to add new ones. I imagine it's just because of the JS error that the form is not rendered correctly.

I tested with 2.359, it's broken. When tested with 2.222.4, the plugin (with same version) was working fine, meaning it's highly likely that the root cause is about a recent UI change.