See https://phabricator.wikimedia.org/T180666. Currently, when you try to delete a repository from the web UI, we give this misleading justification:
Repositories touch many objects and as such deletes are prohibitively expensive to run from the web UI.
This was added long ago in D3822, and was relevant/accurate then, but modern Phabricator has plenty of ways to do slow things in the background. In modern Phabricator, repositories can't be deleted because we (mostly) don't let you delete anything.
(There's a similar dialog under "Delete User" but I'm not sure if it's misleading or not.)