Well sn oob must have put it for some reason. I didn't find anything that's officially documented so my suggestion would be instead of removing directly, raise a query in HI support to verify all implications of removing that filter. Especially keeping your environment landscape in mind.
In my opinion, if you remove the filter it would mean someone can create a archive rule on cmdb itself which the parent node of all thousands of child node or class. That might mess up the whole cmdb data and relations , who knows!
