I've been testing out the user experience for a Site Pages library with more than 5,000 items. The tenant I'm using has the feature to save an existing page as a template. You can then select the template when you create a new page.
However, the option disappears when your library has more than 5,000 items. You can still choose one of the default page templates, but your custom templates aren't shown. Neither is the link to the Templates folder where the page templates are stored.
Using the browser dev tools I can see the following GET request is failing with a 500 error - exceeded list view threshold:
Is this the best place to post this feedback for the Microsoft SharePoint team?