The custom link /exampleis either invalid or it cannot be accessed by anonymous users.

Hello.

I have created a patch to resolve this issue.
As I saw in simple xml sitemap module, when you want to add custom link, will only check if valid it.
I didn't find solution how check url-links by visible an anonymous user (httpClientFactory and curl doesn't work correct in this case). As for me, better allow user to add custom links, created on site, by self.

Added changes to a patch #comment-14894736 with changes test requirements

Again, added changes to a patch #comment-14894736 with changes test requirements

Added changes to a patch 13 #comment-14894736 with changes test requirements.
Added validation if available file on site, if add it as custom link.

Added changes to a patch 16 #comment-14894736 with changes test requirements.
Remove test, that check visible added files.

The latest patch here (2986810-17-change-validation-custom-links.patch) is working as expected for me.

I found this issue when trying to added view page paths (/news) and custom HTML file paths (/custom-folder/index.html) to XML Sitemap. Each time I encountered the error "The custom link /example is either invalid or it can not be accessed by anonymous users." despite that fact that the paths I was defining did exist and they could be accessed by anonymous users.

Applying the patch has now allowed me to save these custom paths, fixing the issue encountered. I believe this work is ready to go.

Thanks