Very similar vulnerabilities have definitely been discovered in other platforms.
This vulnerability is most similar to the object loader vulnerabilities found in Spring a few years back. It is the kind of vulnerability that is occasionally found in Java web stacks.
It is a simpler vulnerability. This is a double edged sword. On the one hand, it is easier to fix (and to be sure we've fixed) than the objectloader-type stuff. On the other hand, it's so easy to reason about and work with that the exploit is straightforward. It was very difficult to find ways to talk about the general pattern of weakness in this code without immediately disclosing the exploit.
The vulnerability is similar in spirit to Python's Pickle, which is also unsafe for untrusted data. A difference between Raila and Django, though: while specific Django apps have had Pickle exposures, I'm not sure Django itself ever did.
PHP has vulnerabilities that are similar in impact to this vulnerability. But there's a big difference between this flaw (and the Python issues) and PHP: PHP grappled for years and years with a publicly known bug class (remote file inclusion) that coughed up code execution. It's not impossible that more RCE flaws will be found in Rails, but it's unlikely to become a class of bug that every Rails developer will need to adopt best practices to stop.
No mainstream web platform has ever survived long deployment in popular applications without some horrible finding. Nobody's hands are clean. It is very difficult to get security right in every single component that a full-featured web framework needs to offer. It only takes one mistake.
You are dead right about deserializers in general.