Editor's Update 2019-12-05 (by @aduth): A placeholder prop now exists for URLInput as of #17846, but is not yet documented. The task should now reflect the need for documentation.

Describe the bug
Similar to other block components like RichText, the URLInput block needs to have a configurable placeholder attribute so custom blocks can set a different value other than the default Paste URL or type to search

It currently does not allow for one:
https://github.com/WordPress/gutenberg/blob/master/packages/editor/src/components/url-input/index.js#L243

Additional context

• Tested on Gutenberg 4.3.0

I also encountered other bugs with URLInput that I'll file new issues for.