Thanks for reporting this! Confirming. Also see Locating broken scripts for more info how to debug problems with a web browser's developer tools.

That specific line was introduced in rMW0813c46daaa7ca1b52008633a5eb8a769df67137 hence CC'ing its author who might know more.

TypeError: editingRestrictionWidget is null[Learn More] mediawiki.special.block.js:70:4
<anonymous> https://www.mediawiki.org/w/resources/src/mediawiki.special.block.js:70:4
mightThrow https://www.mediawiki.org/w/resources/lib/jquery/jquery.js:3534:21
resolve/</process< https://www.mediawiki.org/w/resources/lib/jquery/jquery.js:3602:12

Side question

OOUI is unrelated to this bug. Feel free to ask OOUI questions in a OOUI support forum / mailing list.

BTW the tag system on Phabricator is awful, I picked some tags at random

How to report a bug explains how to set projects. Feel free to discuss specific Phab topics at Phabricator/Help. I entered "user blocking" and I got MediaWiki-User-management (BlockAndNuke is not available on Wikimedia sites and this task has nothing to do with UI-Standardization).

@Tufor The conversion to OOUI was accomplished in December 2017. Are you sure, this has been broken ever since?
We're not offering a HTML forms only variant, as we're providing functionality that can not be resembled in a user-friendly way in pure HTML forms, provides consistent user-interface between platforms, and goes beyond their capability like for example the DateTimeInputWidget in Special:Block or the TagMultiselectWidget in Special:RecentChanges. If you have specific usability questions on OOUI widgets, feel free to file a task against the project.

Change 482001 had a related patch set uploaded (by Tchanders; owner: Tchanders):
[mediawiki/core@master] Add check for editing restriction widget on Special:Block

https://gerrit.wikimedia.org/r/482001

Change 482001 merged by jenkins-bot:
[mediawiki/core@master] Add check for editing restriction widget on Special:Block

https://gerrit.wikimedia.org/r/482001