A few months ago I unsubbed an account and got a very generic survey.
I think that may have been suggestion to players leaving at one time that was part of the process. There have been a number of changes over the years. Some years ago when I unsubbed, the last page gave you a 255 character text box to type in your reason for leaving, which was completely useless. It would have been impossible to turn that into data that could be tabulated.
The most effective way to find out what was bothering your long-term players so much that they left would be a telephone interview. But they would seriously have to want to know enough to spend money to hire a firm to randomly contact players who unsub. They would have to care enough about the playerbase in general, not some “that player doesn’t play competitively, so they won’t be missed”.
Even more effective would be to do advance research on what players will accept and what they won’t. But that would require a full attitude readjustment on their part.