Thanks @kunjal for the suggestion,
It sounds like you're looking for a way to inform new users not to disclose confidential information while they use the Mattermost service.
The announcement banner feature you're referencing is intended for temporary maintenance and quality-of-service announcements for multi-team administration, which might not fit your use case. For instance, you might want to announce that you're upgrading Mattermost to a new version through an announcement banner. While that is shown, new users would miss the banner about disclosing confidential information.
So instead, there can be a couple of ways to accomplish your use case (available in both open source and enterprise edition):
1) If you have a team (or a few teams) your users join when creating an account, you can add a note in the Town Square channel header informing users not to disclose confidential information, and perhaps even include a link to more information.
I've included an example of what this might look like.
2) If there is no central team that new users join, then a welcome message is one long-term solution. This welcome message could include a note about disclosing sensitive information.
We currently have a few community members working on adding matterbot for Mattermost. One of the planned features for adding Matterbot is to allow team and system admins offer a welcome message to all their new users.
The welcome message could be customized and for instance include a note about information disclosure. You can find more detail in the Jira ticket here: https://mattermost.atlassian.net/browse/PLT-1829
Let me know if you have any thoughts on the above proposals? Would either of the two meet your use case?