Personally, I've disliked just about every change they made after initially adding message threads (that was a good change). It's all been downhill since, making it harder to use and harder to find things (though I've not used this latest redesign, as I'm no longer at a place that uses Slack)
Leaving aside feature bloat, who decided that reviving Yahoo Purple as a big new part of the color palette was a good idea? I'm sorry, but to me that just radiates "arrogant and clueless."