Apple pretty much doesn't respond publicly to anything, so if you want to hear something from Apple, you'll have to get a job there.
The implementation in Mail works correctly and is not abnormal in any way or difficult to set up.
You may have one Trash for all of your accounts (subdivided by account), or you can have a trash for every account. You can set it up how you want.
Normally, when Mail sets up an IMAP account, it maps the IMAP server Trash to the unified Trash. Same for Inbox, Junk, Drafts. If the IMAP server names those mailboxes something non-standard, they don't get integrated. I'm sure there could be other reasons why it didn't get automatically mapped, like yours.