The Slack for Nonprofits program offers free or discounted plan upgrades to eligible organizations. Here are the details:
Standard Plan: Slack offers workspaces with 250 or fewer members a free upgrade, and an 85% discount for workspaces above that size.
Plus Plan: Slack offers workspaces of any size an 85% discount.
Organizations that hold a valid charitable status with either the IRS (in the United States), a local tax service/charity commission, or a local TechSoup Global partner.
Also, your organization must not be any of the following: