The feature is still available, but the UI has been updated. If you are creating a new group, you will find the setting in the ‘Automatic’ section of the new Group configuration page. To add an automatic email domain to an existing group, click on the group’s Manage tab, then click the Membership link from the left-side menu.
It is not possible to add users to the Moderators group that Discourse creates for you. To add users to the Moderators group, you need to go to the user’s Admin page and click the Grant Moderation button. The only groups that you can add users to automatically, based on an email domain, are groups that you create. The groups that you cannot add users to based on an email domain are the staff, moderators, and admins groups.
You cannot add a user directly to a trust level group based off their email domain, but you can use custom groups to effect a user’s trust level. This is configured in the group’s Effects section. Users will be granted the trust level set in this section when they are added to the group.