You'd think but sadly passenger manifests are often wildly inaccurate (or at least used to be up to about 6 years ago) and if its a big airport they may not even be saved digitally once the flight departs. The boarding list is of limited further use anyway - as they don't check them off the other end for 99.99999% of flights.

I used to be an aviation crisis responder and you'd think an accurate list of passengers and crew would be very simple once boarding cards were all computerised etc.

But we were often waiting for at least a day if not several for a full manifest and then had to very very carefully data check it for duplicates, mis-spellings and just people missing from the list. We rarely were lucky enough to get the one from at the boarding gate which might have had scribbled pencil additions at the gate for the passenger whose boarding card wouldn't scan, the passenger with a line through as they were bumped to another flight or added by hand onto this one with hand written last min replacement boarding card, and a myriad of other situations we've probably all experienced and never given any thought to. But once the flight departed (the gate not the runway which could be 20min + later) that tatty bit of paper used to just be filed in the nearest confidential waste shredding bag. If the incident happened at departure or very early there was a chance they'd be able to retrieve it. But often it's landing phase that's the most dangerous so it would be long gone when the cleaner collected the shredding waste sack.

The most accurate source was always relatives turning up at the departure or arrival airport and talking to the airline (or usually our staff on their behalf given they'd also just lost friends and colleagues). Part of our team were specialists in bereavement and witness statements (lots of ex cops, Paramedics, counsellors etc) to collect and refine the antemortem details that could help us ID their loved ones physical remains and personal effects and get them home. Mentally a really tough job but with a huge sense of accomplishment and purpose being able to use my skills to help people - I had to give up once I had my own kids as it struck too close to home.