"Deleted" ( http://www.merriam-webster.com/dictionary/deleted [nofollow] ) - means"eliminated", "removed", i.e. it no longer exists. And it is only for convenience of a rush user or (sometimes) occasional technical reason, that those messages are still kept in the "Deleted" folder.
To me, that's an obvious reason.
If you want to perform any operation on them, you must undelete them first.
That's very similar to how "Recycle Bin" behaves in Windows: you need to undelete files and folders before you can looks at what's in them.
I appreciate your perspective, when I first read it I was nodding my head in agreement.
However deleted messages do exist, in spite of what the dictionary may say. And in my case there are reasons for their retention other than the "convenience of a rush user or (sometimes) occasional technical reason". For example, I may consider an email no longer relevant only to discover at some point in the future (before it's gone forever) that I should use it.
Regarding similarities with the Windows Recycle Bin, you don't have to undelete an email to view it and operations on deleted emails can be performed (forwarded, replied, ...) without undeleting them first.
So from my perspective I still feel an option to display deleted emails in AquaMail's conversation view would be nice to have. It'd probably be best to have the default set to not display deleted emails in conversation view if Kostya felt like implementing this idea in version 999999.1.0.
And many thanks to "The AquaMail Forum Team", it's a valuable resource for AquaMail users!
Cheers, Byron