It actually never made its way into the HTML specification. If you look at this from the view of the HTML specification, "marquee" never existed, and there is really no such tag to be deprecated or removed.

On the other hand, this tag was first introduced by Microsoft, and at this point, it is supported by all major browsers. However its support can be dropped any time, since it is not part of the specification. In this sense, you can safely call this tag "deprecated", and try to stay away from it.