It's not just the attributions leaving the main feed. Just a few months ago Twitter was going out of their way to promoting 3rd party clients (in the form of free advertising in the web client, in the space now occupied by Who To Follow). The last few months have seen almost all promotion for 3rd party clients disappear.
Going from attributing every tweet in the main feed and promoting outside clients freely on the sidebar of everyone's account to shunting attributions into a secondary info pane, that's a very large shift.
Going from attributing every tweet in the main feed and promoting outside clients freely on the sidebar of everyone's account to shunting attributions into a secondary info pane, that's a very large shift.