Allo was announced back in 2016. Unlike its popular video counterpart Duo, the app never really took off — so it was no surprise when Google formally announced the app's days were numbered in December. According to a banner on the Allo website, the service's time has come: it's shutting down tomorrow, March 12.
If you're particularly attached to any of the conversations you had in-app, now's the time to download them for safe keeping. To do that, open up Allo, and in the app's side menu, tap Settings then Chat. Here, you'll be presented with the option to export text from your chats as a .csv file, or media attachments as a .zip file.
According to the blog post in which Google announced Allo's closure, the app is being shuttered to focus more resources on the development of Messages, Google's regular ol' texting app. You can read more about the troubled history Allo here.