We are sorry for any disruptions but we are a non-profit with zero service guarantees (in particular for an entirely free service). So intermittent failures of any length must be expected.
If you have an event, where you want to actively partner with OpenSky, then this could be taken into account. We have no way of knowing and with all the people using it, somebody will complain every time it is down for maintenance. Without maintenance, however, the service won't be available for very long.