No, I don't have a citation, but is it so hard to believe that lots of people want an undelete? I accidentally delete emails all the time and I go into the trash and fish them out. Not giving users an undo seems ... unfriendly.
EDIT: I see, it applies to Custom Folders and delete only removes the label; doesn't move it to the trash. Here's what Google has to say about it. Is this a default IMAP behavior or Gmail-IMAP specific? https://support.google.com/mail/answer/78755?hl=en
As for "undelete," I believe the purpose of the Trash folder is to support that. I have yet to find the email client that does not, as a default, store deleted messages in the Trash folder. I am not disputing that people want that functionality, what I am saying is that I do not think people want the behavior that I am seeing.
The fact that GMail treats "delete" as "remove labels" is very problematic. IMAP supports labels, including client-defined labels. Treating folders as "labels" only breaks the abstraction IMAP presents. I suppose this was part of Mark Crispin's gripes with GMail.
I was just trying this, I moved something to trash and then did a 'Delete Forever'. Nothing appeared in trash or all mail again.
[citation needed]
Seriously, I have trouble believing that anyone would complain about "delete" carrying any meaning other than "delete." I also find it hard to take such people seriously, given the existence of a Trash folder as a first stop for deleted messages, and All Mail as a second stop (and what is the default for deleting from All Mail? Having the message come right back to All Mail! Brilliant...).
Thanks for the tip on how to fix this behavior. It is an easy option to miss...