Problem: "rsync -aX remote:/somepath ." fails, if the remote site doesn't support extended attributes. Sample session:
# rsync -aX aix1:/usr/man .
rsync: extended attributes are not supported on this server
rsync error: syntax or usage error (code 1) at main.c(1554) [server=3.1.1]
rsync: connection unexpectedly closed (0 bytes received so far) [Receiver]
rsync error: error in rsync protocol data stream (code 12) at io.c(226) [Receiver=3.1.1]
Since this is a read-only access on the remote side a warning should be sufficient.
The rsync can't speak the xattr protocol needed to make the transfer work and the client has no way to know that in advance. Thus, it dies and lets you remove the option.
Would it be possible to ignore the -X in this case?
Nope, that's the whole point.