I've tried to figure out how to patch it and looked at the docs but it was not clear to me how to do it so I gave up :)

Seems pretty straightforward to me:

$guard = $permissionManager->addTemporaryUserRights( $user, 'some-right' );
// do stuff that requires the rights
ScopedCallback::consume( $guard );

Maybe there should be some generic documentation about the scoped callback pattern that can be linked from methods returning a scoped callback; there's a ScopedCallback wiki page but it's not super informative.

@Tgr Thanks. It's not the docs but my lack of PHP skills :-)

I guessed this also required to specify at the top:

use MediaWiki\MediaWikiServices
use MediaWiki\Permissions\PermissionManager

But I'm not sure.

Change 524068 had a related patch set uploaded (by Legoktm; owner: Legoktm):
[mediawiki/extensions/MassMessage@master] Use new PermissionManager::addTemporaryUserRights instead of hook

https://gerrit.wikimedia.org/r/524068

Change 524068 merged by jenkins-bot:
[mediawiki/extensions/MassMessage@master] Use new PermissionManager::addTemporaryUserRights instead of hook

https://gerrit.wikimedia.org/r/524068