I've been having this problem with push notifications for quite a few apps, but Slack just seems to genuinely do whatever the hell it wants, regardless of whatever I've set preferences to.

Yeah I know what you mean. Like sometimes I'll get messages hours after they are sent. Even when my main computer is off so would be considered "away". Now I just have to frequently check slack and that in itself feels like a dark pattern.

Same here... no clue what's happened over the last few years. Slack used to be pretty reliable when I first used it.