intentional, imapx doesn't support it.

Does it make sense to support it though?

I was looking at this while working on the CamelSettings branch and it didn't look too difficult to get working in IMAPX, with only a trivial change to Evo.  I've had one user tell me he won't use IMAPX because it lacks this feature, but if it doesn't make sense for IMAPX then I'm fine with throwing it out with the older IMAP provider.

Only seems useful when coupled with sophisticated client-side filtering.

That would be surprising if this will start working with a change on evo side only. But in general, I've no objection to have this feature for imapx too, just the opposite, because if you want to replace something completely, by removing older stuff, then you cannot do that if you drop functionality which is useful. That's why I do not see imap being replaced by imapx soon, there are features which are not done in imapx, and nobody cares (real junk/trash folders, for example).