From Mattermost v7.10, using Mattermost in a web browser or the desktop app, you can set a timed reminder on posts made in channels, direct messages, and group messages.
Hover over the message you want to remember, select the More option, then select Remind. Select a time period or enter a custom date and time. You’ll receive a direct message at the time you’ve chosen, with the body of the message you wanted to be reminded about.
Once you’ve been reminded about a message, you won’t receive another reminder unless you set one.