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.
Slack's been pretty clear that they don't want their product to be used for this use case, and have even nudged some larger "casual" communities and open-source projects off. So it's not surprising that they plan their feature roadmap according to what (they think) will be useful to businesses.
Given that they steadfastly refuse to implement an ignore user feature while at the same time they've implemented mandatory channels, of course the signal-to-noise ratio falls and people just ignore Slack. I know I do.