+1 to this ticket.It's so frustrating it's more that 2 years old, has a trivial fix (that doesn't affect default operation) and still wasn't fixed.
Log levels were invented long ago. But os-prober thinks that it's OK to log debug output with notice priority (I'd be fine with that if it at least allowed to disable this stupid behavior).
If there is any way I can help with getting this fixed I'd be glad to help.