Rarely are passengers told that their bags are not onboard when they depart. Just makes them angry/miserable for the flight and there's not much that can be done anyway to change that. It's usually not enough ground staff for the demand or the aircraft is too heavy.
However, when it's happened to me, I did get an email from BA informing me that my bag was not loaded. (This was in June 2021). They still took 3 days to get the bag to me. That was disappointing.
Nowadays, I rarely check in a bag when I am work and I use Apple AirTags to track them.