You must've joined after it was removed. http://itsjerryandharry.com/threads/ignore.88710/ I found an older post so this thread isn't as needed, though still open for discussion.
There's really no point in adding it/putting it back in anyways. Usually if someone spams you it's warnable. So just report them and be on your way.
As @Reebal said, if someone continually aggravates you after you have told them to stop or leave you alone it's against the rules. Apart from that why else would you want /ignore All it does is create more problems when it isn't really needed For example, staff get messaged by someone to help them, they /ignore the person and the person doesn't get help. Even if the system only works for no staff there are other problems that are still to be dealt with. So all in all it isn't really needed at the moment.
It's not always about whether or not someone is doing something to you specifically or something against the rules. If they're talking to someone and the chat is getting filled with them, they're just having a normal conversation but you don't want to see only them. Just little things like that. I don't know if the good would outweigh the bad, but I definitely don't think that having it added back on would be as big of a problem. The command just made it so that you can't see their stuff in all chat. Like you had said they could just make it so that you can't ignore staff.
the addition of the /ignore commands can decrease warnings, sure, but this causes problems where players can ignore staff, which I assume won't be allowed for obvious reasons. there are certainly more problems than just the one listed.
Phew, I was starting to second guess my memory xD I'm genuinely not sure how many problems it would create/solve, that's why this is in server discussion and not suggestions.
Maybe they should be able to re-implement it so you cannot ignore staff. To be honest I don't like some of the stuff kids have said to my daughter over the past couple of weeks. Granted, staff has been on top of the warnings/mutes in these cases. But in all seriousness some of these kids are vile.