As far as a standardized nummeric, Its not, but apparently there's no conflicts with it's use as RPL_CHANNEL_URL as explained
HERE. As far as it being a bug, due to it not being standardized, and it'd be simple enough to code a handler for it I don't feel that this falls into the catagory
I haven't found a way to test, but the following should mimic outputting the text as though it was native:
raw 328:*:{
if ($1 == $me && $2 ischan) {
echo -tc info2 $2 * Channel URL: $3-
halt
}
}