It's meant to partially break YAML deserialization :) My apps do care about YAML, so I've an interest in cleaning this up. Is there some unintended consequence? You can still instantiate some Ruby classes (Regexp, Symbol etc.) in the YAML loader, or you can go through @@tagged_classes and pick out any other types you don't want.
But by taking out Object, YAML is only left with a whitelist of types that are safe, anything else will get turned into a YAML::DomainType.
What I mean by that is, this workaround breaks application code that depends on other portions of Rails that use XmlMini. In exchange, it allows you to potentially expose YAML to HTTP requests, which is still an extremely bad idea.
I don't see why YAML is a dangerous serialization format - the other type deserializations in the code seem sane and limited enough. (I wouldn't use YAML over e.g. JSON these days but I'm fixing up quite old projects)