Is the Baltimore Catechism still a valid resource to teach the Faith?
Yes, the Baltimore Catechism is still valid. Obviously it is missing anything from the Second Vatican Council but it still contains all the essentials of the faith. It can be used to learn the basics of the faith and can be supplemented with some more up to dates materials. Some of the wordings may no longer be in use and the Church may have elaborated further on some points over the past century but there is no error contained in it.
In fact, when I’ve taught in my parishes’ schools I’ve always used it as my base text.