How does grouping by repository solve that problem?

Like, is "Students" a repository and not a project? Can you put "Students" into a project and filter open results that way? Where do repositories come into play?

You can also already filter by repository. Why do you need to group by repository?

The goal is to sort the contributions of one person who committed to a lot of different repositories, to get an overview of where they contribute. For example, that allows me to identify what repos are on my expertise domain so I can review there / who I can contact to unblock situations for other repos.

At Wikimedia for example, we have 1500+ repositories. It's not really convenient to try to guess what repositories a new contributor will target.

Would having the repository on the "card" solve the issue, albeit not as perfectly? My guess is for instances that 1.5k+ repositories, always knowing the repository on a diff card is likely useful, where just bucket queries are likely narrowly useful.

Aye, that would allow to visually scan the cards, and to understand what repo is concerned by each change.

"Show repo on card" is T10992: Allow Object Item List be customized for display, which is T418: Integrate CustomField into more interfaces

Also, T10850: Search revisions by "Repository Projects" is related (And rather recent, WMF might not have it yet).

... but I would be very happy to get just the repo on the card now, if we can upstream it until T418.

If bucketing is really what you're after, you can have it by just dumping this in your extensions:

<?php

final class DifferentialRevisionBucketByRepository
  extends DifferentialRevisionResultBucket {

  const BUCKETKEY = 'repository';

  public function getResultBucketName() {
    return pht('Bucket by Repository');
  }

  protected function buildResultGroups(
    PhabricatorSavedQuery $query,
    array $objects) {

    $viewer = $this->getViewer();
    $grouped = mgroup($objects, 'getRepositoryPHID');

    $groups = array();
    foreach($grouped as $repo => $revisions) {
      if (!$repo) {
        $name = 'No Repository';
        $repo = 'null';
      } else {
        $name = $viewer->renderHandle($repo);
      }

      $groups[] = $this->newGroup()
        ->setName($name)
        ->setKey($repo)
        ->setObjects($revisions);
    }
    return $groups;
  }
}

But the resulting UI is a little poor if you have more than maybe a dozen revisions in the results, and there's no (?) way to join this with the "Action Required" bucketing so I don't think it's really worth it.