As per the documentation : https://docs.mattermost.com/deployment/sso-saml.html#saml-single-sign-on-e20 SAML currently does not support automatic deprovisioning, and is only possible if AD/LDAP sync is enabled.

For environments where AD/LDAP is not possible, it would be great if SAML could also automatically deprovision users from Mattermost as well.