I am to speculate that they are going to use this as an excuse to let people go without doing mass layoffs and having to pay severance. Training AI is just an excuse.
(I work at Meta)
The only report chain based permissions were around distribution lists which were just some powershell scripts that walked AD every night. These also got used for security groups to gate access to some things. Be default, calendars were visible to all authenticated users unless you made them private or individual events private. The meeting tool leveraged this for example.
I was working on corp email (among other things) there from 2009 to 2016, so I can't speak for now.
It’s only once the business is having a cash crunch or will no longer need to hire competitive candidates that they start letting people go without severance.
Tell that to Elon Musk and Twitter employees.