There's various cases of not great looking callables when EchoAttributeManager::ATTR_LOCATORS is used.
For example, in OAuth there is EchoAttributeManager::ATTR_LOCATORS => [ Utils::class . '::locateUsersToNotify' ],.
AbuseFilter has (more brackets than I'd expect), but it works:
EchoAttributeManager::ATTR_LOCATORS => [ [ [ EchoUserLocator::class, 'locateFromEventExtra' ], [ 'user' ] ] ],
So the OAuth example should be updated to EchoAttributeManager::ATTR_LOCATORS => [ [ [ Utils::class, 'locateUsersToNotify' ] ] ],
From what I can see, the format isn't well documented. Not in code, or on MediaWiki.org...
Other usages around are fully stringified...
EchoAttributeManager::ATTR_LOCATORS => [ 'EchoUserLocator::locateEventAgent' ],
Would be nice to update most (all?) of these to use ::class or similar.