i3 supports some EWMH. To find out which are likely supported, you can make yourself a quick list with the following list in the source directory:
grep -Eor '_NET_[^ ]+' .
As for the bug, you are referring to, it looks like the software wants to know the height of the title bars et cetera. i3 takes the perspective that this is none of the business of programs. i3 is the benovolent dictator for this kind of information, and apparently does not see any benefit in sharing its knowledge through the EWMH. This would only be of benefit for applications that pretend to be their own window manager. The i3 way of getting this type of info is through IPC-calls.
That being said, it could very well be that if you provide a patch to support EWMH-hints that you need, that it might be accepted. It could be wise to discuss concrete hints on the mailing list before making work in vein.
i3 supports as much of EWMH as people find useful enough to implement