I think lots of freelancers do, but basically you use whatever communication platform the client wants to use. There are certainly issues over which one might refuse to bend to a client's wishes, but "what channel we will use to communicate" should not be one of them. They're paying you, so if they like phone calls, or video meetings (on the platform of their choice, usually zoom or google's equivalent), or email, or meeting in person, I do that. Lately, nobody much wants to meet in person, so it's mostly been video meetings or email or phone.
But, if/when they want me to use Slack, I do that.