I like some of the reddits. /r/programming is pretty good, though with significant overlap with HN.

Slacks are fantastic, if you can find focused ones with good moderation. For example, the Rands leadership slack is, bar none, the best place to go for engineering management advice: https://randsinrepose.com/welcome-to-rands-leadership-slack/

I presume the same is true of focused discords, though I haven't really found any of those that stand out.