The "you can take it to private channels" can play out either way I guess: it can be a completely valid replacement of using company channels, but it is an additional hurdle if you have no signals to go by who to talk to. And if you actually have a problem with work channels getting out of hand, I feel like it's a gamble to hope it works better in the dark.

If my employer told us we had to move the more social/less technical chat channels outside of company infrastructure, I'm not sure that'd be better overall. And sort of odd to go with each new hire "oh and btw there's this second chat without the bosses, come join".