The log message error ignored appears after some delivery failures. What does it mean?

This message is written when Exim fails to deliver a bounce message whose age is greater than ignore_bounce_errors_after. It indicates that the failing bounce message has been discarded. The same message is written after failed deliveries when a filter file uses the noerror feature when setting up a delivery, or if a router has the setting

errors_to = <>

Both of these specify that delivery failures are to be discarded.