IdentityServer supports single signout via WS-Federation. Your RP would need to make the signout request to IdentityServer to have the user logged out of IdentityServer and any other RP that had authenticated with IdentityServer via WS-Fed. Here's a snippet of how your RP would make such a request:
var sam = FederatedAuthentication.SessionAuthenticationModule;
sam.SignOut();
var fam = FederatedAuthentication.WSFederationAuthenticationModule;
var signOutRequest = new SignOutRequestMessage(new Uri(fam.Issuer));
// optional -- put your RP's URL here
// signOutRequest.Reply = "https://localhost:44321/";
var url = signOutRequest.WriteQueryString();
Response.Redirect(url);