
I think you should use singular name for your custom content types in SharePoint online communication sites.
When you add the custom content type in your SharePoint online list and click on "New" button to create a new list item based on custom content type, it shows the name of your content type.
As you are creating a single list item, content type name should be singular.
For example:
It should be shown as:
New > Announcement
And not like:
New > Announcements
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