Yes, but only in Production (ie: non neutralized) databases.

Administrators can create and update a System Parameter each time a MOTD is needed:

Key:

sysadmin.message

Value:

{
    "warning_type": "user",
    "message": "<div class='alert alert-info'>Odoo will undergo system maintenance on
<strong>January 31st</strong> between 8pm and 9pm</div>"
}

The available warning_types are either user or admin to indicate who sees the message. You can decorate the message with the alert class together with alert-info (blue), alert-success (green), alert-warning (orange) or alert-danger (red).

To remove the message, you can set the Value to:

{}

This is available for Odoo 16.0 and higher.

Result: