We've managed to get "Sign in with Slack" to work using the following settings:
Client authentication: Request body (client_secret_post)
Authorization endpoint: https://slack.com/openid/connect/authorize
Token endpoint: https://slack.com/api/openid.connect.token
Userinfo endpoint: https://slack.com/api/openid.connect.userInfo
Scope: openid email profile
Linking strategy: Link on email. Create the user if they do not exist.
Reconcile lambda: Default OpenID Connect Reconcile provided by FusionAuth