Feature summary (what you would like to be able to do and where):
In partial blocks form, allow naming a non-existent page as a block target

Use case(s) (list the steps that you performed to discover that problem, and describe the actual underlying problem which you want to solve. Do not describe only a solution):
If a user repeatedly creates a page, the only remedies are either to create a user-specific AbuseFilter that gets triggered on page creation (wasteful, not available to wikis without AbuseFilter), or to block the user from editing an entire namespace.

Benefits (why should this be implemented?):
This can stop a user from creating a specific page, without blocking their access in any other way.

Additional Information
T2674 enabled Partial Blocks, but it was designed such that "Page blocks can only be set for existing pages only, with validation required in the input field."

T194697 now allows multiple concurrent blocks.