I recently (last week) received my data dump following a data access request, from AF. As far as old bookings are concerned, they don't have anything earlier that that which is displayed in my online list of past reservations despite flying with them since before the turn of the century. I was at least hopeful they would have data from the start of Flying Blue in 05. But no, nothing.

So if you have access to old email accounts, that's probably the best option, or paper records.