Checking for notarization on each launch was introduced in catalina. Older versions have trustd, but it was only used for the gatekeeper checks added in 10.8.

`/usr/libexec/trustd` exists on Mojave, too. There's a (very unhelpful) manpage.

I think you were just lucky to not open non-Apple applications during the outage.

I ran into this on trying to load a new video file on VLC, with Mojave, so I guess it's not just apps, but maybe any new file load.

My 2018 MPB on Mojave had some serious issues launching apps for a little while yesterday (3PM central) afternoon. It seemed to resolve within an hour though. Not sure how that lines up with the outage described here.