By default when session timeout occurs, Liferay will redirect to home page. In case we need to redirect to custom url we can do that by implementing SSO interface as below :

1. Create an OSGi module and place a new Java class into a package in its src folder.

2. In the @Component annotation, add service=SSO.class and implement SSO Interface as below :

import com.liferay.portal.kernel.security.sso.SSO;
import org.osgi.service.component.annotations.Component;

@Component(
        immediate = true, service = SSO.class
    )

public class SSOImpl implements SSO {

    @Override
    public String getSessionExpirationRedirectUrl(long companyId) {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public String getSignInURL(long companyId, String defaultSignInURL) {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public boolean isLoginRedirectRequired(long companyId) {
        // TODO Auto-generated method stub
        return false;
    }

    @Override
    public boolean isRedirectRequired(long companyId) {
        // TODO Auto-generated method stub
        return false;
    }

    @Override
    public boolean isSessionRedirectOnExpire(long companyId) {
        // TODO Auto-generated method stub
        return false;
    }

    
    

3. We need to override the below two methods :

@Override
    public String getSessionExpirationRedirectUrl(long companyId) {
        return "https://liferay.dev"; //URL where redirection should take place after session timeout
    }
@Override
    public boolean isSessionRedirectOnExpire(long companyId) {
        return Boolean.TRUE; //To Enable Session Timeout Redirection
    }

Note - Above example is done on Liferay 7.3