php - How to add a custom "under maintenance" page?

Solution:

Have you tried this: "Website Temporarily Unavailable" error page

Solution:

In your app's settings.yml set these:

all:
  .settings:
    module_disabled_module: security   # To be called when a user requests 
    module_disabled_action: disabled  # A module disabled in the module.yml
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In the module's module.yml set this:

prod: 
  enabled:     false
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This will disable the your module for prod environment, but it will still be available for dev and test environments.

Hope it helps. Regards, Radu.