Slack is now rolling out a new field for profiles that will let you list your preferred pronouns right under your name and job title. Once filled out, your pronouns will appear both on the mini-profiles you see when you click on someone’s name and in their full profiles.
Note that your Slack profile may already have a pronouns field, if an administrator created a custom field for your company’s employees. (When I went to check The Verge’s Slack, I found we already had one.) Slack says administrators can continue using that custom field or choose to turn on this new one, though the new dedicated pronouns field has the benefit of appearing more prominently in your profile.