Some misguided software is using it for "progress reporting". OSC 9 has
a decade plus history of being used for notifications but because
Windows Terminal decided to use it for progress reporting, without
bothering to do research about conflicting existing uses, systemd of all
things wants to adopt it!! Crazy world we live in.
Since actual notifications are highly unlikely to start with 4; we
hopefully don't break any real world use cases and also don't let
systemd spam our users with endless notifications.
Fixes#8011
847433dba2