During Dynamics CRM 2016 days (now known as Dynamics 365 CE), Announcements were used to broadcast important news to all the CRM Users in an Organization.
However this feature is presently not available in Dynamics 365 CE (as of June 2020)!

Zap Notifications / Alerts App provides a similar feature which allows adding announcements in Dynamics 365 CE / Power Apps.

Lets consider the Use-Case: Administrator wants to notify all users of a scheduled network downtime!

Below are the steps required to add announcement using Zap Notifications App:

Step 1: Browse to General Bell Notification and click “New”
Add Name, Start Date, End Date and Severity Level

Step 2: Add the message

Points to Note:

  1. Message can be plan text or HTML
  2. Message supports Merge fields… Eg you could add fields from User entity
  3. Message can be added for multiple languages (Notifications App is multi-language compatible)

Step 3: Add the Notification Audience (Users, Teams)

Step 4: Publish The Notification!

Now based on start date and end date (configured in step  1)… the users would be automatically shown the notification (as in below screenshot):

Also the notifications are in real-time and show-up with a notification count (next to bell icon) as soon as the notification is published (as shown in below screenshot):

For more information, please refer below links:

Zap Notifications / Alerts App feature page: https://zapobjects.com/apps/zap-notifications/
KB Article: How to configure Trigger based Notifications
For list of all Zap Apps check-out: https://zapobjects.com/

Please note that Zap Notifications App works with Dynamics 365 CE as well as Power Apps (Model-Driven)!