This sort of information is available on the Data Hub documentation page, found here: OpenIFS Data Hub. For planned downtimes the box under the heading "Latest Updates" will contain information about when the Data Hub becomes unavailable for a longer time period, or if a major upgrade has recently been implemented.
For shorter or unplanned downtime periods, such as during ECMWF System Maintenance Sessions or when any hotfixes are applied to the Data Hub, the status banner at the top of the Data Hub dashboard page will inform the users whether any request submission is currently possible.
Please note that the input data for initial data requests is dependent on retrievals from the MARS archive. If the MARS archive is degraded or access is temporarily not possible due to maintenance, this will also affect the performance of Data Hub requests which will remain queued during MARS downtime.
For the current status of ECMWF systems other than the Data Hub, such as the MARS archive and others, please refer to the public ECMWF Service Status portal here: https://status.ecmwf.int/
Best regards,
Marcus