DynamoDB: How to model data that's shared and queried by all users?
I'm intrigued as well.  :)  I have a use in mind that would likewise have userID as the PK.

Sounds like you could put an 'announcements.json' file in an S3 bucket, then have your application read announcements from that file.

S3 can handle 5,500 read requests per second, according to:
https://aws.amazon.com/about-aws/whats-new/2018/07/amazon-s3-announces-increased-request-rate-performance/