If you want to disable Sitecore Identity Server and use previous version of Sitecore authentication on your website, the best way to do this is by enabling the config file
You can copy this file to another folder under
\App_Config\Include\ and remove the
.example extension from the file name. Once this config is activated, it will apply the following changes:
- Set the
Enabled property of the
SitecoreIdentityServer provider to
false so that this federated identity provider is not executed during login and logout process
loginPage URL for websites
admin to their initial values
After this you can stop the Identity Server website without impact on CMS functionality. Here is Sitecore documentation page for reference.
To answer your original question, the impact will be the following:
while Identity Server is enabled, Sitecore pipelines will try to generate login and logout URLs using the Identity Server provider. For login functionality there is a fallback: if the Sitecore instance cannot reach the Identity Server during the first sign-in after Sitecore has started up, it uses the
/sitecore/login page as a login page fallback. It looks like there is no fallback for logout, that's why you get the Not Found error.