I'm interested in hearing from someone at FSF (and I used to know someone, but I don't think he's there any more), who can tell us how often this has happened. I can't imagine it's a frequent occurrence.

I love how small of a world this site is: https://news.ycombinator.com/item?id=43784538

As I implied in my top level comment, it should happen more often than it likely does. If you work on a commercial project with any GPL code ask your test group who has done that and when - if you don't see a lot of hands go up then your test group isn't doing their job. (if you are only automated tests, then I assume you have an automated test to send this letter and verify the response)

Yeah I sort of hoped it would cover that bit not just the author's foibles at writing a letter.