> If you have to ignore your colleagues you have far worse problems than Slack not having an "ignore user" feature.

Try working at a large enough company. Or, try using any number of bots. I'm in a few mandatory channels of zero relevance (ok, mute) and a few of high relevance that are punctuated with lots of bot chatter I'd like to ignore. And, sure, there are a few coworkers I'd prefer to ignore.

Even IRC will let you manage notifications with more granularity than Slack will.