Harry_Fryer - you're from MS - so I'd like to think you could reach out to the Yammer team.
>Is there a release date for Storylines?
Later in Fall 2022.
>We were shocked when Viva Engage was released to all our users before we had the chance to communicate it.
There were some messages in the M365 Admin Center before anything launched.
>Will we be able to prevent Storylines from releasing, so that we can stage deploy it when we are ready?
Firstly, Storyline is available now in Public Preview and it default ships off. There's also a toggle to prevent it from appearing when it reaches GA. See the blog post announcing the public preview.
https://techcommunity.microsoft.com/t5/yammer-blog/storyline-is-now-open-for-public-preview/ba-p/3621068