This was wholly a display change within the web (and possibly mobile) apps. All of the outgoing headers were identical, and third-party clients displayed them as is. At least we know from OP's quoted response that the change was deliberate.
Interestingly I've just gone into two of my catch-all folders and I'm not seeing this behaviour any more. None of the previously affected messages has the "me" in any column or header.
Three possibilities spring immediately to mind:
- FastMail have deliberately removed this feature due to backlash (I haven't seen much actual backlash in public spaces, but maybe they had lots of support tickets...)
- They have tweaked it so it only applies when there are multiple recipients in the relevant fields. A quick check of a message sent to me and multiple others suggests this isn't the case but I only checked a single, older, message.
- Mitigating the recent DDOS/outage has necessitated a rollback to earlier code without the change, and the change will come again with the next update.
Or, of course, something I haven't thought of. Going to keep an eye on this though. For such a minor change it was disproportionately irritating and I'm glad it's gone.