You can communicate with clients through Slack or another instant messenger only if they have purchased the Custom Communication Channel add-on.
Here are some required guidelines for the Slack channel:
You should check Slack messages first thing in the morning and also before the end of the day. You do not need to be readily available to communicate 24/7.
Approvals and deliverables need to be documented via email, Miro, client portal, etc. - NOT on Slack
Client must understand that while they own the channel, Designity still has rights to access the thread history & attachments
Client may provide assets through Slack, however, you should only be providing screenshots or references, never the deliverables