It's difficult but I'd avoid places like that. If your team won't allow you to close slack for an hour you have serious communication and planning problems. Anywhere I've worked what I do is message anyone who might need vital information from me then just close slack for an hour, do my work and check back in. When I get pulled up on it I give my explanation about focus, communication etc. If they don't like it then I leave. I'm glad it's a seller's market.