"Deficient" is a big understatement.
Like others here, I archive my emails (including SENT mail) On My Mac to avoid leaving it on the server. I do this by moving messages to local folders, including messages in the SENT folder.
With Mail being so crippled, I can no longer find sent messages when I need to refer to them, because there is no way to sort by "To" anymore.
It's not even possible to isolate a recipient by making a "Smart" folder because there is no filter on "To," only on "From."
Mail has been getting progressively worse and worse with each new version. The removal of these columns is a terrible decision and it must be reversed.
I have to say, this situation is eroding my trust in Apple. First iPhoto is replaced by Photos, losing my careful organization of photos. That's a BIG deal. Photos need to be archived, organized, and managed long term. Now, Mail is becoming unusable. Again, that's a BIG deal. Email needs to be archived, organized, and managed long term. Apple is making decisions with far reaching consequences without any thought.