It replicates timestamps, attributes, DACL, SACL, group and owner info, as well as alternate file streams. Replication of some of these is off by default though.
It can follow symlinks, reparse and mount points, but doesn't replicate them.
I see. Note that normal file attributes != extended attributes. Which one are you referring to?
(For symlinks/reparse points, you'd probably have to provide an option on how to replicate them, since there isn't one clear solution. But it's not impossible to do something sane either. Link Shell Extension may have inspiration on various methods you could use.)
Extended attributes have been deprecated by Microsoft quite some time ago in favour of ADS. It's not hard to add support for replicating them, but there's literally no demand for that.
Not sure where you got that idea (link?) but EAs are used in WSL, among other things. Meaning you won't be able to copy WSL file systems properly right now.
As a general rule, you should avoid any information loss regardless of anyone's opinion on the matter. No one is going to think "ah well, Microsoft/etc. doesn't like this feature so it's ok my backup is messed up".
I don't have a link ready, but this was explicitly mentioned in one of TechNet articles on the subject dating back few years.
EA being used for WSL (Windows Subsystem for Linux) - not much of a surprise, really. It is a square peg in a round hole in many aspects. As I said, there's zero demand for replicating EA, WSL or not.
It can follow symlinks, reparse and mount points, but doesn't replicate them.
It does not know about hardlinks.