Last year we added the multi-select feature on list of issues, which allows to make the same change to many issues in a list.
But this feature was missing from the list of Github notifications. It's a list of issues, but not the same kind, because not related to a single repository.
So we added the multi-select feature, but it only allows you to mark as read/unread and active/inactive many issues at once, which can really be useful if you have many notifications.
For example, to mark all the notifications from a specific repository, before that you had to do it issue by issue.
Now it's a lot more faster: filter the list on a repository, then activate the multi-select feature, select all the issues, and mark them all as read!