We haven’t removed this feature. It’s just not added yet. CalDAV consists of dozens of extensions. So called “managed attachments” are one of them.
As of now it’s not a standard yet – it’s just a work in progress draft that Apple already implemented.
We plan to add support for managed attachments, too. But because we are dedicated to open standards we have to do it properly and not only hack it in there to support just one vendor. So this will take some time for the feature to evolve to a solid standard. We definitely have an eye on it.