A Jira issue (https://mattermost.atlassian.net/browse/MM-4815) implemented a modal pop-up that prompts users to reset their presence to "Online" when they first open the client in cases where they previously set their status to "Away" or "Offline" in the UI.

Once you select "Do not ask me again" you are never asked again. As a downside, there is no way to reset this prompt. You're stuck with your decision forever... unless you can hunt through the Mattermost DB to delete the right value from the right table.

Users should be able to reset any "do not ask me again" prompts associated with Mattermost in their user profile settings.

GitHub issue: https://github.com/mattermost/mattermost/issues/28014