SLO (Single Logout URL) is a way to logout not only from the SP (== the application that authenticates users via SAML), but also from the IdP (the service it's authenticated against). See also https://www.identityserver.com/articles/the-challenge-of-building-saml-single-logout.
Even though there's no full support for it in passport-saml, it seems as if it's basically doable: https://github.com/node-saml/passport-saml/blob/eacbbbb3919ec0cf767f81696f05870eacaae237/src/strategy.ts#L232
I'd offer myself as tester for that :)