Not sure whether Slack communities are less or more active, but what I'm sure is that chatting hardly works as a means to convey knowledge. It's neither rewarding nor persistent. I mean, you spend your time for free answering some (usually) easy to google questions and soon your answer is lost in the countless logs and you have no way to refer to it later like it never happened.

All popular and large Slack communities (the same is true for IRC channels) I used to be a member of are very much alike – hundreds of people just sit there busy with their lives just in case somebody would post something interesting, and just occasionally there're small bursts of activity to educate another newbie who did @here in the channel. And even when (oh, miracle) there does occur some conversation you realise you have no time to follow it as it happens and later you're just to lazy to find it and try to make some sense out of it. So, first you turn off notification distractions. And second, after a while, you realise you haven't poked there for weeks and finally just remove it with no regret.

Discord. It was made mainly for games and it has exploded the last few years for pretty much about everything. I was introduced to Slack because the programming language I was using had its official channel there. They had also an irc channel but people were not willing to use it. Then I made a discord server mainly because I was using Discord for Unreal Engine , which has to this day the most active community there according to my experience. Without me realizing it the main dev of the language I was using abandoned Slack and used my Discord server. I made him admin and the rest of the community reluctantly followed cause being coders saw no point using a game orientated chat client but a year later not only the Slack server was almost completely abandoned but not one even mentioned Slack or IRC.

People get swallowed in the garden walls of 'social' apps and just browse, mostly interract via <3 and Likes, and don't really have opinions of their own anymore. Also typing, you know, it's a lot easier to tap an icon to mean something than type it awkardly.