I manage and develop many stores and have noticed this app update issue showing up everywhere.
I'm hoping the app developers get the message as reaching out to all of them across all stores is going to be a nightmare task. Shopify have told me that it's all taken care of, but this is a warning for those of you unaware.
Here is a list of apps currently raising alarm bells
Fraud Filter, Metafields Editor, Facebook Shop, Stamped.io Reviews & UGC, Bizzy Social Proof, and Smart RSS Feed, Zendesk, Embedded JS Buy SDK,
I'm the Apps team lead at Pixel Union - we're behind Bizzy Social Proof. I can confirm Bizzy Social Proof will continue to work on the deadline, no action required on your part as far as that app goes.
There was a bug in the notification system that erroneously listed Public apps (including Bizzy) in the Private Apps warning - this should now be fixed and the list of apps in that banner should be only Private apps.
Hope that helps!