As mentioned in our blog post here from earlier this month (read the blog here), we will be trying new features out with Windows Insiders in the Dev Channel through a variety of ways. This means sometimes you might see a feature or change, and sometimes you might not - and sometimes you might see a feature or change and it reverts back as we try things out. As mentioned in our blog post for Build 22557, the new Task Manager design is only available to a subset of Insiders right now as we monitor feedback before making it available to all Insiders in the Dev Channel.
And one additional note to address some of the other questions/suggestions in the thread: the Update Stack Package is not in any way related to this feature being enabled (or not enabled). :)