Especially bad with instant messaging clients.

We used to have this — a separate window with contact/conversation list and another one with conversations themselves so you could have multiple open at a time and quickly switch between them instead of hunting in the list every time. Everything else (profiles, settings, etc) also opens in separate windows. Nothing is modal unless it really has to be. These days, everything is modal. Profiles are modal, settings are modal, sending media files is modal. There are a few exceptions, for example, recent versions of Telegram Desktop let you open some things in separate windows using cmd+click, but still, compared to what we used to have, downgrade would be an understatement.