8.18.0
This is by design for specific (OOTB) alert definitions.
Pre-defined alerts can be modified or cloned, except for those defined against all adapter instances. These alerts exist because they are not tied to a specific adapter type—for example, alerts that trigger when an object is down, regardless of the adapter type, as long as the condition matches. However, you can create and modify your own custom alert definitions.
While modifying OOTB (Out-of-the-Box) content is possible, it is not recommended since any changes will be overwritten during the next patch or upgrade.
The best practice is to clone the OOTB content before making customizations, as cloned copies will persist even after an upgrade.
If you select any of the following object types—vCenter Server, Datacenter, or vSphere World—as the Base Object Type, it will, by default, apply to the entire vCenter and other vCenter instances if multiple vCenter integrations are enabled.