They all seem to be either bouncing explicitly or silently not going through now (0 open/click rate), and they had been working since they first introduced the feature. I know they aren't giving users the option to choose a proxymail.facebook.com address anymore in the newer Auth box. Thanks.
I can answer the specific question I asked so long ago. I think Facebook must have had a bug for a while that broke the *@proxymail.facebook.com email integration, and it was during this period that I asked my original question. However, looking at our email logs, I can see that we have had people click on links in email sent to these Facebook proxy emails at the end of October, so whatever the issue was must have been resolved, and I believe if you have a Facebook proxy email, you can safely send to it.
That being said, of course the individual user may have revoked the email permission (in which case their proxy email will go to nowhere), and my impression is the same as sfussenegger that Facebook is no longer allowing users to use proxy email addresses. I haven't seen the option to anonymize my email in a FB permissions dialog in a long time...