Hum indeed that changed in 4.0/3.10:
On Linux, both for GTK+ and Qt, the default modifier key for rectangular selection is now Alt. This is the same as Windows and macOS. This was changed from Ctrl as window managers are less likely to intercept Alt+Drag for moving windows than in the past.
Ideally we'd have a setting for this, but yeah we should revert to the previous behavior.