But in general I agree with the sentiment that this feature should have been easy to make optional (none of the underlying data changes, it's just a UI control), so I think Slack deserves the scorn for this one.
Related, rhe absolute one saving grace feature that I commend reddit for is old.reddit.com. It may be "ugly", but it also lets me be extremely fast when browsing (not a ton of extraneous whitespace like I can't stand in the new interface). IMO lots of "designer-led redesigns" killed many a previous chat forum (looking at you Slashdot). A common thread is many of these redesigns are built to appeal to a less expert-level user. But by doing this in a way that is not optional, you are guaranteed to piss off the folks who are the biggest users of your product. It is no wonder they are vocal!