If you want immediate answers to pressing questions, you have to just make friends with knowledgeable people so they don’t mind you interrupting them by directly messaging them. When I have time I check this BAR website pretty frequently, when I don’t have time I don’t check and generally don’t want to be bothered. So I wouldn’t use a BAR Slack, even though I use similar systems (Teams, Skype) for work.
I’ll go out on a limb and say most experienced reefers also wouldn’t want to use it for it’s intended purpose- notifying people immediately every time someone has a question. If it isn’t so urgent and can wait, we already have this forum and direct messaging on the forum for that. And if most experienced reefers aren’t on there, who is giving the advice? I just don’t see a non-annoying use case personally.
Also as mentioned it diverts the conversations from the site, and I like to see those conversations even if I don’t have anything to add, so I’d feel a loss there.